Intervention under Rule 19 requires a legal interest that is actual, substantial, material, direct, and immediate in the matter being litigated; it is not proper when it seeks to inject an independent controversy, would unduly delay or prejudice the adjudication of the original parties' rights, or change the nature of the action, especially when the intervenor's rights can be fully protected in a separate proceeding.

The dispute originated from the sale of two lots (Lots 986 and 991-A) owned by the late Estanislao Miñoza to the National Airports Corporation (NAC), MCIAA's predecessor, in the late 1940s/early 1950s for the Lahug Airport expansion project, which allegedly included an assurance that the original owners' heirs could repurchase the lots if they were no longer needed for that purpose. The airport expansion did not proceed, leading the purported heirs (represented by Leila Hermosisima) to seek reconveyance based on the alleged buy-back option over forty years later.

• Leila Hermosisima, purportedly representing the heirs of Estanislao Miñoza through his children Adriana, Patricio, and Santiago, filed a Complaint for Reconveyance against MCIAA, seeking to repurchase Lots 986 and 991-A based on an alleged buy-back option agreed upon when the lots were sold to NAC (MCIAA's predecessor) in 1950.
• Before MCIAA could present its evidence, the respondents (Heirs of Filomeno T. Miñoza, Pedro T. Miñoza, and Florencia T. Miñoza) filed a Motion for Intervention.
• Respondents claimed they were the true and legitimate heirs of Estanislao Miñoza and his wife Inocencia Togono, alleging their ancestors were Filomeno, Pedro, and Florencia.
• Respondents alleged that Adriana, Patricio, and Santiago (predecessors of the original plaintiffs) were not heirs and had fraudulently executed an Extrajudicial Settlement in 1958, adjudicating Estanislao's estate to themselves.
• Respondents further alleged that the sale of the lots by Adriana, Patricio, and Santiago to NAC in 1958 was fraudulent, deceitful, done in bad faith, and void ab initio.
• The initial Complaint-in-Intervention lacked the required verification and certification of non-forum shopping.
• Respondents subsequently attached a Complaint-in-Intervention with the required verification and certification to their Motion for Reconsideration filed before the RTC after the initial denial of their intervention motion.

• Allowing intervention would unduly prolong, delay, complicate, and change the nature of the original reconveyance proceedings.
• Respondents' legal interest in the subject properties is merely contingent or expectant, not direct and immediate, as required for intervention.
• Respondents' rights concerning heirship and alleged fraud can be fully protected in a separate legal action.
• The initial failure to include verification and certification of non-forum shopping in the complaint-in-intervention was a fatal defect warranting dismissal (though the Court later found substantial compliance).

• They are the true, legal, and legitimate heirs of the late spouses Estanislao Miñoza and Inocencia Togono.
• The original plaintiffs (represented by Leila Hermosisima) are not related to the deceased spouses.
• The Extrajudicial Settlement executed by Adriana, Patricio, and Santiago in 1958, and their subsequent sale of the lots to NAC, were fraudulent and void ab initio.
• Determining the true heirs is a focal issue, not merely collateral, as it impacts the validity of the original plaintiffs' claim and potentially MCIAA's acquisition.
• Allowing intervention would avoid multiplicity of suits.
• The procedural defect of lacking verification and certification was cured by their submission attached to the Motion for Reconsideration before the RTC.

• Whether the Court of Appeals erred in reversing the RTC's orders and allowing the respondents to intervene in the reconveyance case.
• Whether the respondents possess the requisite legal interest (actual, substantial, direct, immediate) in the matter in litigation (the reconveyance based on a buy-back option) to justify intervention under Rule 19 of the Rules of Court.
• Whether allowing intervention would unduly delay or prejudice the adjudication of the rights of the original parties or improperly expand the issues by injecting independent controversies (heirship dispute, fraud).
• Whether the initial lack of verification and certification against forum shopping was fatally defective or was cured by subsequent compliance.

• The Supreme Court GRANTED the petition filed by MCIAA, REVERSED and SET ASIDE the Court of Appeals' Decision and Resolution, and REINSTATED the RTC Orders denying the Motion for Intervention.
• The Court held that while the respondents substantially complied with the procedural requirements for verification and certification against forum shopping by submitting them with their motion for reconsideration, the intervention itself was improper.
• The respondents' claim involves determining the true heirs of Estanislao Miñoza and proving fraud in the 1958 extrajudicial settlement and sale, which are independent controversies separate from the original issue of whether the heirs represented by Leila Hermosisima have a right to repurchase the properties based on the alleged buy-back agreement.
• Intervention requires a legal interest that is direct and immediate in the specific matter being litigated; injecting the complex issues of heirship and fraud would unnecessarily complicate, delay, and change the nature of the original reconveyance suit.
• Allowing intervention is not an absolute right and rests on the sound discretion of the court; the RTC correctly exercised its discretion in denying intervention because it would retard the principal suit and the respondents' rights could be fully protected in a separate action.

• Intervention (Rule 19, Section 1, Rules of Court): Defined as a procedure allowing a third party with a legal interest in the matter in litigation, or in the success of either party, or an interest against both, or who may be adversely affected by a property disposition, to join the action with leave of court. The Court applied this rule to evaluate respondents' motion, considering the nature of their interest and the potential impact on the proceedings, ultimately finding intervention improper in this case.
• Legal Interest Requirement for Intervention: This doctrine requires the intervenor's interest to be actual, substantial, material, direct, and immediate, not merely contingent or expectant, meaning the intervenor must stand to gain or lose by the direct legal operation of the judgment. The Court held that respondents' claims of heirship and fraud, while potentially valid, were not direct and immediate to the specific issue of the buy-back right being litigated in the main case, but constituted separate issues.
• Substantial Compliance with Procedural Rules: This principle allows courts to relax procedural rules like verification and certification against forum shopping in meritorious cases. The Court acknowledged that respondents cured the initial defect by submitting the required documents later, demonstrating substantial compliance.
• Prohibition Against Injecting Independent Controversies via Intervention: Intervention is generally not permitted if it expands the issues, changes the nature of the action, or introduces matters distinct from the original dispute. The Court applied this by reasoning that the heirship dispute and fraud allegations were independent controversies that should be litigated separately, not injected into the reconveyance case.
• Judicial Discretion in Allowing Intervention: The decision to grant or deny a motion for intervention rests upon the sound discretion of the trial court, considering factors like the intervenor's interest, potential delay or prejudice, and availability of other remedies. The Supreme Court affirmed the RTC's exercise of discretion in denying intervention as rational and proper under the circumstances.

• "Intervention is a remedy by which a third party, not originally impleaded in the proceedings, becomes a litigant therein to enable him, her or it to protect or preserve a right or interest which may be affected by such proceedings."
• "The interest contemplated by law must be actual, substantial, material, direct and immediate, and not simply contingent or expectant."
• "In general, an independent controversy cannot be injected into a suit by intervention, hence, such intervention will not be allowed where it would enlarge the issues in the action and expand the scope of the remedies."
• "Intervention is not intended to change the nature and character of the action itself, or to stop or delay the placid operation of the machinery of the trial. The remedy of intervention is not proper where it will have the effect of retarding the principal suit or delaying the trial of the action."
• "Furthermore, the allowance or disallowance of a motion for intervention rests on the sound discretion of the court after consideration of the appropriate circumstances. It is not an absolute right."

• Altres v. Empleo (G.R. No. 180986, December 10, 2008): Cited to explain that non-compliance with verification is not necessarily fatal and can be corrected, and that certification against forum shopping signed by one plaintiff may suffice under justifiable circumstances (substantial compliance).
• Asia's Emerging Dragon Corporation v. Department of Transportation and Communications (G.R. Nos. 169914 and 174166, March 24, 2008): Cited for the definition of intervention.
• Metropolitan Bank and Trust Co. v. Presiding Judge, RTC Manila, Br. 39 (G.R. No. 89909, September 21, 1990): Cited for the definition/nature of intervention proceedings.
• Alfelor v. Halasan (G.R. No. 165987, March 31, 2006): Cited for outlining the requirements for intervention under Rule 19 and defining the necessary legal interest (actual, substantial, direct, immediate).
• Nordic Asia Limited v. Court of Appeals (451 Phil. 482 (2003)): Cited for the point that allowing persons not parties to intervene unnecessarily complicates proceedings.
• Big Country Ranch Corporation v. Court of Appeals (G.R. No. 102927, October 12, 1993): Cited for the principle that intervention is improper if it changes the action's nature, expands issues, or retards/delays the principal suit.
• Quinto v. Commission on Elections (G.R. No. 189698, February 22, 2010): Cited for the doctrine that allowance/disallowance of intervention rests on the court's sound discretion.

• Rules of Court, Rule 19, Section 1 (Who may intervene): This provision was the central rule analyzed and applied by the Court to determine the propriety of the respondents' intervention.