Throughout most of his late-teenage years, Asriel dedicated a lot of his time to missions and training. Even consulting other mentors such as Kakashi Hatake and Kurenai Yuhi to help him develop even further. At some point, he even used the Multi-Shadow Clone training technique that he heard of from his own mentor, Yamato. With that kind of training and obsession to become stronger, Asriel further developed his usage of not only his Ninjutsu, but his visual prowess as well. This lead to him unlocking the Armored Susano'o, that carried a spear that could split into two javelins. He trained nonstop, went on missions constantly, pushing himself until he was on the verge of fainting. Which Yamato always saw in Asriel when they first started training. He always wants to get stronger, any chance he gets, even at the expense of his own rest. Saying he can just recover later, and that later soon became never. It was only until Yamato and Kakashi had to tell him to get some rest, that he finally did decide to do so.
Asriel fought in the Fourth Great Ninja War, facing off against threats such as the reanimated Second Mizukage and Third Raikage, where he helped Naruto Uzumaki. He also fought former Akatsuki members, such as Deidara and Sasori, where he used his Susano'o and Shadow Clones to his advantage. That advantage allowed the nearby sealing unit to seal the two. But later in the war, when Might Guy was down from using the 8-Gates, Asriel stepped in to buy extra time for Naruto and Sasuke. Facing against Madara Uchiha, he walked in with full confidence, pushing the Ghost of the Uchiha to a corner with his constant usage of Clones, Okikaeru, Susano'o, Ninjutsu, and at times; Flying Thunder God. However he ends up losing this battle, as Madara took advantage of his Limbo Clones and struck Asriel where he couldn't anticipate it. But even as he lost, he had a smile on his face, knowing the real trouble; Naruto and Sasuke, are back on the field.
As Kaguya Otsutsuki faced off against Naruto and Sasuke within her Lava Dimension, as she paused to gaze at the eyes of Naruto and Sasuke with tears in her eyes. Asriel saw the emotional opening and decided to capitalize on it. Interfering with their battle, Asriel offered Kaguya a deal, where if he were to convince her that Humanity was worth keeping around, she would spare the world. But should he fail, she could take the world as she pleases, and return all chakra to herself. This was a risky move for everybody involved, especially Asriel as he dropped his guard entirely to show Kaguya he had no hostile intent. Black Zetsu immediately tried to intercept that proposal by immediately dismissing it, however Kaguya was not listening to him either. Intrigued but also skeptical, Kaguya paused.. really paused. Before raising her hand and launching an All-Killing Ash Bone towards Asriel, made to purposefully miss, she simply wanted him to react so she has an excuse to keep fighting. But surprisingly, Asriel did nothing, only closed his eyes as the Ash Bones missed him by a centimeter. Seeing how serious, and interesting Asriel was, Kaguya used Ame no Minaka to return herself and the others back to the real world. Team 7's reactions were... mixed to say the least. Even Naruto had doubts, and when asked about this, Asriel simply replied;
"The pros outweigh the cons, at best she fully stands down, at worst we just have to keep fighting"
While he knows deep in his heart that's not the reason he wanted to spare Kaguya. He just doesn't want to condemn someone to an eternity of being sealed, without giving them a proper chance to redeem themself. Even when Naruto's reasoning failed, Asriel didn't believe Kaguya was beyond saving. Enraged, Black Zetsu proceeded to throw insults at not just Asriel, Naruto and Sasuke, but also Kaguya. To which he will find it to be his last mistake, as Kaguya expelled him from her sleeve and immediately killed the Zetsu with an Ash Bone.
After the Fourth Great Ninja War, after the Infinite Tsukuyomi was released, Asriel and Kaguya went on to travel the world for 4 years. Asriel staying true to the deal, he did his very best to try and convince Kaguya that Humanity is worth having around. And while everyone rests, recuperates or atone for their past sins, Asriel carries the weight of the world on his shoulders as every day he will spend for the next few years, will be spent on trying to prove Humanity's worth in the eyes of somebody like Kaguya Otsutsuki.