The King of Fighters All-stars is the Best Free-to-play Gacha, I have ever played. There are Tons of ways to get your favorite Character OP with great cards setup to have some fun in the early game. You can actually beat some high levels in the story, with some low level characters, if you play smart or super tryhard. Some of the King of fighters' characters get at least one outfit, that will make you say "I wish this a mod or an official design in the main games".

There are some awesome collabs like Gintama, Samurai shodown, Tekken, Street fighter, DOA 6 and Virtua fighter. That if you catch the event, you will most likely get the one you want the most, just by playing the game. And the collabs aren't just cheap wallpaper, no, I think the collabs are great and offer the best of the feature franchise, This game allows having Devil Jin, Jacket Jin and Flame Pants Jin in one team, Honoka doing Kyo's DP and Benimaru's rekka, Gintama character's making jokes about KOF lore and games like they belong in the franchise. That makes doing the events feels no like a chore, more like a grind.

This grind is great when the events and early game rewards you, with a lot of resources to make you stronger and rubies to gamble. I have spent 2 years grinding(2020-2022) in this game, and today when I logged in, it was still worth it, since my op's characters stood the test of time and multiples updates of the game since I stopped playing. As you can tell, there's a point where the grinds become a chore with no great reward, and great gameplay ideas just basically become cookie clicker. Free to play becomes extremely hard when you have to depend on events and collabs to give you rubies.
Don't get wrong, this game is super Free-to-play friendly and I recommend it.
If you want to start on the gacha games, this is a great starting point, to learn habits and master the free-to-plays ways you may need on your others gacha games.