Marvel Puzzle Quest has a neat aspect of its design that boosts new players, hampers journeyman players, and encourages players to advance and spend money. Most events give a boost to certain characters and require that they be in your team.
If you are a new player, this is awesome. You are level 3, but you get a buffed level 15 Dr. Doom to fight alongside you. This is why Black Widow is especially great in the early game: your buffed NPC is out there tanking and doing extra damage, while charging up your support characters’ abilities.
If you are a journeyman player, this is lousy. You now have some strong characters, but not all of them, and this event now forces you to use a lower-level character who may not synergize with your best characters. The same mechanic that made you stronger in the beginning, when you did not have anything better than a buffed level 15, weakens you in the mid-game, when you definitely do have something better.
Players with a substantial investment in the game will have a strong version of most characters or the resources to boost one up to a decent level for what feels like a very small investment. I have dropped thousands of ISO (in-game currency/xp) into characters who I must use during events, because I can get 10-15 levels on those guys or 2-3 on a level 50-ish two-star character. And as the game will happily tell you, you can always spend more money to level your characters really quickly.
This design encourages players to spend by giving them trial-sized portions of more characters. Also, some missions during events will require a particular character but not provide it for free; invest in more characters or miss the rewards for those missions.