Tierno is watching Y try to persuade X to leave his house when Xerneas and Yveltal appear in Vaniville Town in the middle of battle. Y quickly saves Tierno, Shauna, and Trevor, but Tierno is horrified to see Vanville's dancing school be destroyed. After the legendary Pokémon have gone, he and the others set off for Aquacorde Town.
Once there, they stay at an inn. X refuses to go in, instead staying in his tent on Y's Rhyhorn, and Tierno comes over to him and shows him the two remaining starters from Professor Sycamore. X becomes infuriated with him, grabbing him by the collar, but Tierno is uaffected. When his anger is not returned, X lets go of Tierno and disappears back into his tent, refusing Chespin and Froakie.
Tierno goes back to the room he shares with Shauna and Trevor. Unfortunately, the inn is actually run by Team Flare, and Tierno, Shauna, and Trevor are trapped inside until X's Chespin blows a hole in the wall and Y's Froakie makes a cushion of bubbles for them to jump out and land on. The five escape from Team Flare and decide not to trust any strange adults again.
They arrive at Santalune City next, where Tierno buys food and supplies for the group. Viola finds them and invites them to her gym. Everyone but Shauna agrees, and the four go to the gym while Shauna runs off.
At the gym, Tierno goes through photos of Vanville Town's destruction with Trevor and Y. Later, Tierno and Trevor come across Shauna wielding a sword at Y and an unconscious Viola. Celosia explains that Shauna has been possessed. Tierno has to dodge as Shauna goes on a rampage and then faints.
Tierno is kind. His dream is to be a dancer.
Corphish has been seen standing beside Tierno.
Tierno is childhood friends with them.