Harry Potter - According to Word of God, seven is a lucky number in the Potterverse. Hence we have: seven books; seven years at Hogwarts; six Horcruxes plus the fragment of soul still in Voldemort's body or so he thinks - Harry is actually the seventh Horcrux, which proves his downfall. "Isn't seven the most powerfully magical number?"
IT : The last third of the book continually drops reminders that for their particular brand of magic to work, all seven members of the Losers club have to be present. In the film version, they actually refer to themselves a "Lucky Seven."
Bluestreak (later Silverstreak) was Diaclone Car Robo #7, but was not so popular as Prowl (Car Robo #13).
On the Microman side, Micro Change #7 was designated as one of the evil Decepticons!
In BIONICLE, all related characters were released in groups of six. This was reflected in the story as well, with six tribes, six leaders, and six Toa to protect them. Later on down the line, a seventh Toa is added into the mix. Lucky number, right? Wrong. Apparently, Toa groups can be any number. Two, six, fifteen, a hundred anything. Six is preferred, but it can be anything else. Except seven. Seven means bad juju is about to go down.
All over the place with Punch-Out!! Wii's version of Aran Ryan. He always gets up from a knockdown on the seventh count, you can hit him seven times when he's stunned, and sevens pop up in his challenges in Exhibition Mode. He's Irish, so it's a "lucky seven"/"luck of the Irish" gag.
Final Fantasy VII (figures) causes a character to go berserk if their HP hits 7777, attacking all enemies up to 63 times for 7777 damage per hit: the only enemies that can withstand this and not die are the Bonus Bosses. The Lucky Seven concept continues in the prequel Crisis Core, where getting three Sevens on the DMV (the battle system's slot-machine gimmick) will level up your character.
In Final Fantasy VI, Setzer has a slot-machine-like attack, and getting three 7s kills all enemies.
In Final Fantasy X-2, getting all 7's on the Lady Luck dresssphere's slots skill will result in an attack called "CONGRATS!", which inflicts a One-Hit Kill to all enemies and yields many bonuses for the party.
The seventh Touhou game is the only one to feature the "Supernatural Border" system, which can and will save your life on occasion. It is also the first game in the series to have visible hitboxes.
"7th", or Fiora, the seventh playable character in the party, as this is referred to by users on boards. Saying why this is important is incredibly spoileriffic.
Mario Kart 7 features an item called "Lucky 7", which grants the player seven items at one time.
Kingdom Hearts has the Seven Princesses of Heart, a group of seven young girls whose hearts are completely devoid of darkness.. When brought together, they open the doorway that leads to Kingdom Hearts, which is the very reason why the Big Bad is after them. ...At least in the first game. In Kingdom Hearts 3D: Dream Drop Distance it's revealed that their hearts were born from the seven fragments of light from the x-blade. When faced against thirteen darknesses, namely Xehanort's true Organization XIII, the resulting clash will ensure the recreation of the x-blade, which is exactly what Xehanort seeks.
The Trickster class in Fire Emblem Awakening learns a skill called Lucky Seven. It raises hit and avoid rate by 20 for seven turns at the start of a battle, both stats returning to normal afterwards. Fittingly, Trickster is the default class of Anna, whose base luck stat is already high and is sure to get even higher as she has one of the highest Luck growths in the game. In the same game, Cordelia is recruited in Chapter 7, her starting level is 7, and her birthday is July 7th.
In Cookie Clicker, most of the numbers associated with the bonus Golden Cookies are strings of 7s (the bonus to cookies per second they can give is X7 and lasts for 77 seconds, the upgrades for them have costs in strings of 7s, the achievements for them involve clicking 7, 27, 77, 777, and 7,777 of them).
As a homage to this trope, the highest jackpots on a slot machine is often assigned to three 7s (which, perhaps not coincidentally, add up to the ideal number in blackjack).
Seven is the most likely roll of a pair of standard 6-sided dice. In craps, you can win by rolling a seven (or eleven) first, and lose by getting seven on any subsequent roll. The fact that seven is the most likely sum for the outcome of rolling two dice (with a probability of 1/6) is likely the origin of the whole "seven is a lucky number" superstition.
Zayin is the 7th letter in most Semitic abjads (writing system). It is the source of both the modern "7" and "Z", and also meant "sword" (several proto-versions look like basic swords, and the Arabic derivation bears a slight resemblance to a scimitar).