F-Rank Ox? I'm Thunder

Hale Dunn awakened an F-rank beast soul, a yellow ox, and the people around him called him Old Ox for it. He was also carrying a system out of an earlier life, which nobody knew about. His girlfriend ended things with him, and the break set off a second beast soul, thunder, that had been sitting in him. He teamed with a girl holding an SS-rank ice phoenix and another holding an A-rank thunder leopard. The three of them went through the trials in the cavern, then a beast soul exchange meeting, then an old sealed realm, and his level went up at each stop. The Dunn family and three other houses ran a scheme against him in that period, which the three of them broke apart. Forces out of the dark world crossed into the continent and he fought them with the other two.