After abandoning Mud, a male Mudkip and his original starter Pokemon for being weak and losing to Norman, Ray went back to Professor Birch's lab and received Tree. During a training sencession where Gabe and Mud were training, Ray saw that his former Mudkip had evolved into a Marshtomp and after seeing how strong he had gotten, Ray offered him a place on his team. Gabe told him to back off and that Mud was her Pokemon now. Ray sniffed and said that Mud still belonged to him and told Mud to follow him, but after seeing how cruel his original trainer really was, Mud refused to go with him clinging to Gabe. Enraged, Ray tried to send out Tree to hurt Mud to force him to go with Ray. But Gabe had Pik use his thunderbolt on Ray and Tree sending them running away. Tree was seen again with her trainer during the Hoenn League where she was used to battle Eve, but lost to her. Ray yelled at her afterwards threatening to abandon her too. Gabe heard what Ray had said and yelled at Ray saying that he doesn't deserve to have Pokemon because of the way he treats them. Ray snapped at Gabe to mind her own business. But Gabe threatened to take Tree away and train her herself with more loving care. Ray refused to give up Tree. After Ray lost the battle with Gabe, Ray recalled Tree and Daniel and set off to continue his journey. Tree reappeared with an redeemed Ray. She was used to battle Gabe's Marshtomp, Mud in an friendly battle. During the battle, she evolved into Groyvle.
Tree was mistreated and abused by Ray and this has made her stressed and mean to other Pokemon. She tries to be loyal to her trainer, but he often pushes her away and threatens to abandon her. She was deeply afraid of Ray, when he redeemed himself, she was kind to him and he was kinder to her and other Pokemon. She evolved because of the new bond they had together.