Warning: This entry contains major spoilers for plot for the second game of the Trails series up to and including the ending. As this game has not yet been released in English if you are waiting on the release do not read any further. Translations from this game done by myself and Misa.
Once the party finishes up the battles with the other Enforcers on the Axis Pillar they are getting towards the end when they run into Leonhardt and finally have to battle him. And this time, he has no plans to hold back at all with them. He summons two archaisms to join him in battle and they fight.
The party eventually wins that tough and grueling battle, yet he's still standing and claiming that was merely a warm up to him. Joshua then steps forward and says he'll fight Leonhardt alone. They have quite an interesting battle and during it discuss things related to their past which leads to an argument about Karin.
Eventually Joshua puts all of his power into one blow taking the risk that if it doesn't work, he'll be defeated, but if it does work, he might just win. It does work, and he disarms Leonhardt. The battle stops then, even though Joshua is so winded Leonhardt could easily pick his sword up and kill the boy. But he's so amazed by Joshua's skill in even accomplishing that much that he lets Joshua have the victory for managing to disarm the Sword Emperor.
They talk a bit and Joshua hugs him. He's surprised by this, of course. The two brothers hadn't really spoken in friendly terms in five years, after all. Then Joshua talks about how much he worried about Leonhardt even when he couldn't remember who he was and then at that point Leonhardt returns the hug, amidst cheering from the party.
Leonhardt tells them all that he will be leaving Ouroboros since Joshua gave him the answer he wanted about humanity's strength. He explains about Weissmann's plans and is going to even join the party. However, that gets interrupted by Weissmann who comes in and hits Leonhardt with a lightning art, knocking him to the ground and causing him to lose consciousness. Joshua's Sacred Mark is activated and Weissmann takes the boy away.
Before they can decide what to do, two Black Dragions come to attack the group, a parting gift from Weissmann. Leonhardt wakes up and tells them to go after Joshua. The rest of the group had arrived, they will handle the Dragions with him.
Once Estelle's group takes off to go after Weissmann and Leonhardt is able to stand again, he summons a Black Dragion of his own to turn the tide of things with them and then takes off after Joshua. He arrives just in time to find the party unable to get past Weissmann's barrier. He flies down to interrupt. After a discussion with Weissmann and finding out he was the cause of Hemel being destroyed and Karin dying, he shatters the unbreakable barrier like it's no stronger than an eggshell. Weissmann is infuriated by this and tries to deal with Leonhardt by making the barrier to explode. Leonhardt ends up falling quite a distance, his Black Dragion shatters into pieces.
Joshua and the others try to help him, but he tells them to continue the fight and not worry about him. He can't even move at this point and knows that even if he were to survive by some miracle, he'd likely never be able to stand on his own again. But he's badly enough injured that he's not going to last much longer and only can hope he can live long enough to see Weissmann defeated by his little brother and say goodbye.
The party does defeat Weissmann and Joshua comes running to his brother. He says he'll treat Leonhardt's wounds, but Leonhardt refuses the assistance saying he's not going to live. Joshua is very upset to hear this and Leonhardt says he's still a crybaby like when he was little. Joshua agrees and says he needs Leonhardt, but Leonhardt instead asks Estelle to promise to support Joshua. Better than anyone, Leonhardt knows his little brother is likely to react badly to this and may get himself killed without someone to support him.
Estelle promises him. Then he talks about how he finally understands why Karin was able to smile as she died. He's smiling too and then he's gone. Joshua freaks out at this point, but his brother no longer can hear him.