Warechu is extremely afraid of Plutia, due to the trauma she inflicted in her HDD form when he teased her. Ever since that incident, Warechu tries his best to avoid angering her, not wanting to suffer more torture again, such as ratting out his own allies or complying with the CPUs' orders.
Not much interaction is shown between Warechu and Peashy. It can be safely implied that Warechu dislikes Peashy, due to her ripping out all of his mouse fur.
Warechu is smitten with Compa, to the point where he will do whatever he asks her.
When the Seven Sages go and kidnap children from the Planeptune Basilicom, Warechu finds Compa. Not wanting to damage his image as a mouse mascot, he tries to reassure her and believes her voice is that of an angel. Later on when he is beaten up by Iris Heart, Warechu is surprised to find he is still alive and realizes that he is being tended to by Compa. In the ensuing interrogation, he refuses to leak anything to the CPUs, but when Compa asks him to, he immediately leaks all evidence he knows.
Warechu doesn't seem to have anything special with Rei. He does join her new Seven Sages group in the true ending of the game though, suggesting that he either has nothing to do or supports her. He is also scared when she gets angry, comparing her change in voice to that like Plutia's.
Warechu views Arfoire as a friend. This is shown when he covers her escape when Plutia arrives, "incapacitating" Arfoire and he asks her to leave. In the true ending of the game, Warechu also works for Arfoire on her eggplant farm, packing eggplants for sale.