Litten doesn't even have a depressing backstory. It only distrusts humans because that's how Litten are. That said, I wouldn't mind a non-starter being the ace, like Leavanny and Krookodile were.
Leavanny was never an ace.. >.> Krookodile perhaps, but even then, its "big" battle at the league was vs. Sawk and Leavanny (ironic..) did quite some damage to it already. I know it also KO'd his Liepard, but we don't go around saying that Gible (who had a similar feat vs. Conway) was Ash's "ace" in Sinnoh. Yes, it defeated Iris' raging Dragonite when it evolved, so it beat a pseudo - and Brycen's Beartic, but not much else. It was definitely strong, but I wouldn't call it the ace.
Even Pignite had credible wins of similar caliber - so it's really up in the air (Lenora's Herdier, Saemus' Heatmor and Emboar, Roxie's Scolipede, Cameron's Hydreigon and Ferrothorn). I honestly think that Unova had a case where its "ace" was a starter stuck in the middle stage. Krookodile was strong, but in my eyes, Ash relied more on Pignite in terms of being the ace in the hole.
Meaning that of the writers wanted, they could fully evolve Rowlet and make Torracat the ace.
In terms of strength and role on the team:
Pikachu = Pikachu
Decidueye = Krookodile
Lycanroc = Gliscor
Torracat = Bayleef/Pignite <- imo the aces of Johto/Unova (before Quilava).
Dewpider/Araquanid = Corphish
Hakamo-o = Hawlucha
Like I said above with regards to Krookodile, in this case, Decidueye would be strong, but Torracat being the one that Ash relies on.