Zero broke through Rank-D - and was feeling like he was invincible right now. I mean with his new power he could literally kill 5 Zero before his breakthrough.
The first thing he thought of was battling against Eleonore who is in Rank-C.
He thought that if he could take on Rank-D while he was Rank-E then he should be able to take care of Eleonore who is Rank-C after he became Rank-D -.
Adding to the feeling of invincibility he felt, he also needed to test his new power. Eleonore was perfect since he could defeat her and end his trauma and also because she was Rank-C.
Battling Eleonore, he could test his new profound power and end his trauma, it was killing two birds with one stone.
He tried to run around and move his body and noticed the difference in speed and control of his mana.
His skill was more smoothly executed and his strength increased by a lot. He spends the rest of his day trying to figure out his new strength.