Not quite a Wild Card, the Hidden Agenda Hero is almost certainly on the side of good. He regularly helps The Hero, expecting nothing in return, demonstrates definite good-guy behavior while avoiding actions which would denote a bad guy...and yet he never explicitly states what his motivation is for doing so, and there's evidence that he's up to something the Heroes don't know about.
Unlike the Aloof Ally who may also be a mysterious hero, the Hidden Agenda Hero is far from being aloof and instead regularly interacts with The Hero and his gang, either being a member of their group or readily accessible when The Hero needs help. When eventually asked why he or she his assisting them, he often covers it up with Blatant Lies or something believable but untrue, if he doesn't manage to squeeze out of answering altogether. Usually he expects, or asks, that you simply trust him. What he is really up to and why he hides it differs from character to character, but common reasons involve not wanting to get others involved in his problems, having sensitive plans which would be disrupted or in danger were the heroes to know about them, or for some reason simply not being at liberty to speak of it.
Compare Guile Hero. Compare Mysterious Backer, which is a species of Big Good, whereas this trope is for someone of a rank/authority/power/ability closer to that of those s/he helps. Contrast Hidden Agenda Villain who besides being villainous, also tends to be even more mysterious than this character as well as a Big Bad. Rather the villainous analogue for this trope is closer to the Dragon with an Agenda. Also compare Enigmatic Minion.
- BlazBlue: Remix Heart: One of Mai's friends, Kajun Faycott, is actually an agent of Sector Seven who answers directly to one of its chief scientists, Kokonoe. While she's always helpful to Mai and her other friends, she's secretly using them to try to find Grimoires (in an attempt to help with Mai's "super taste" problem), in hopes that she might find the fabled Crimson Grimoire for her boss.
- Urahara Kisuke from Bleach. There is almost never any doubt that he is on Ichigo's side, helps him constantly and is accepted by the Heroes as a valuable friend and ally, but at any given point in the show we almost never know what he's really up to.
- Scarlet Lady promotes Marianne Lenore into one. Rather than waiting until Season 3 to show up, she appears during the denouncement of "Mr. Pigeon", introducing Xavier to a pigeon sanctuary and striking up a friendship with him. A glimpse into her inner thoughts shows that her kindness has ulterior motives: having seen that the Ladybug and Black Cat are active, she intends to scout out potential candidates for other Miraculous, in hopes that doing so will enable her to meet her lost lover Fu again.
- Denis Arilan in the Deryni works. As a member of the Camberian Council, he keeps them informed of events around the Haldane royal family. The Council wishes to exercise control over the Haldanes and the transmission of the Haldane powers. Arilan, with his thorough arcane training and a seat on the King's Council, is their inside man for things like the ritual Kelson organizes to trigger some of his uncle's powers in The King's Justice. Kelson and his courtiers resent the high-handedness the Council displays, and once they know of Arilan's position they don't entirely trust him. The other Councilors have reason to be wary of Arilan as well; he occasionally takes Kelson's side against them.
- Verin from The Wheel of Time falls under this trope. Until Book 11, The Gathering Storm, Verin was always a suspicious character, with a number of indications that there was more to her than met the eye, but she always seemed to be working for the Light, and not for the Dark One. The true secret of her character is revealed in Book 11: she is The Mole inside of the Black Ajah, as she reveals after taking a quick-acting poison to overcome the magical vow not to give up their secrets "until the hour of her death".
- Auron from Final Fantasy X is undeniably good yet undeniably mysterious throughout most of the game.
- The King of Fighters: The hero for KOF 2003-XIII, Ash, never seems to have any reasoning for competing in the tournaments. Whatever his goal is, he needed to beat up Those from the Past (the villain group in the game) and steal the Sacred Treasures off Chizuru and Iori. In the last game it's finally made clear: Ash is revealed to be the Big Bad Saiki's descendant, and Ash wants to stop him from getting the Sacred Treasures, as well as to ensure his adopted sister Elizabeth is safe. Ash saves the day by making Saiki trapped in the present, which kills both him and Ash with time paradox.
- This is a staple of the Zero Escape series:
- Unlike some of the more mysterious characters of Nine Hours, Nine Persons, Nine Doors, Seven is pretty much the only reliable team player you've got. He claims to be suffering from amnesia, leaving him with no possible agenda. He really a helpful guy and on your side ... but the amnesia schtick is strongly implied to be a Wounded Gazelle Gambit.
- The only characters in Virtue's Last Reward who don't fall into this category are Quark and Dio. Figuring out how to get everyone else to own up to their part of the Big Secret makes up most of the game.
- In the finale, Zero Time Dilemma, both Sigma and Phi know the rough outline of what's about to take place. This is an unusual case, as their strange behavior makes perfect sense to the player, but is suspicious as hell to everyone else.
- In The Order of the Stick, Hayley comes off as incredibly greedy and even seems to cultivates that image, while hiding the fact that she's saving her money to pay a ransom on her father. (Played With, though: while she does intend to use the money to free him, she always planned to just steal it back afterward.)
- Shayera "Hawkgirl" Hol in Justice League. It is never exactly explained what a Thanagarian is doing on Earth, and the question is quickly forgotten thanks to her contributions to the League's success. However, it is eventually revealed that she is an advance spy for the Thanagarian military.
- While Cassidy Williams of Scooby-Doo! Mystery Incorporated is clearly an ally of the Scooby gang, she's also clearly hiding something — namely, that she was Velma's counterpart in the previous incarnation of Mystery Incorporated, and doesn't want anyone else to fall into the same trap she and the previous incarnations did.