This is Exiled Sage, Know the World.
The wizard Haru spent his days being abused by the hero, his childhood friend Leticia, when one day while being scolded he thought to himself, "This is no good". And thus began Haru's adventure to regain his true self! The curtain unveils on an other world "redo" fantasy life!
The protagonist, Haru Traitor, is a wizard in a party of adventurers. At the start of the manga, his party is standing in a forest and he's being lambasted by the Hero of the party and his childhood friend, Leticia, who's depicted with blonde hair in the promotional image above.
Haru remembers a time when Leticia was much nicer to him, but now she talks down to him like he's a useless idiot whose presence she barely tolerates. The other three members of the party all idolise her, so no one comes to his defence. She threatens that if he doesn't listen to her, he'll be kicked out of the party, and he takes that as his cue to walk out on the party there and then, ignoring her protests for him to wait.
He subsequently visits the adventurers' guild, where he meets an adventurer named Alice Spryte, who's depicted with red hair in the promotional image. She asks if he's interested in forming a party with her, but he isn't sure he'd be of much use to her, as he thinks he's weak due to Leticia's put-downs. He shows Alice his adventurer's card, which lists him as being only level 7, whereas Alice is level 22 and Leticia is level 55.
Alice thinks they should try going on a quest together anyway, which they do, and he shocks her with how easily he destroys a 'King Wolf' monster with fire magic. He thought he was using only beginner level fire magic, but she points out that the King Wolf was a level 30 monster and that there's no way it would've been destroyed by beginner level magic. He also uses healing magic, which she says can only be used by priest classes.
Haru begins to realise that Leticia had been psychologically manipulating him to make him think he was weaker than he was, and when they return to the guild, he gets his adventurer's card renewed for the first time since it was issued to him five years earlier, and it turns out that he's actually level 82, meaning he's more powerful than Leticia.
Leticia later confronts him, telling him that he has no right to leave her since he belongs to her. When he rebuffs her, she draws her sword and attacks him, but he says he's done holding back and defeats her with one attack.
