Taiju and Senku have been friends since childhood. In the past, Senku was protected by Taiju from bullies. This led Senku to develop a deep sense of respect and trust towards Taiju even though he is an airhead. When Senku was depetrified, he did everything he could to survive but admitted he needed Taiju and his endurance to overcome the obstables to come.
Yuzuriha and Senku are friends through Taiju, Senku knows how deeply Taiju loves Yuzuriha and respects his friends love interest. He is even willing to sacrifice himself to protect her and abandons the depetrification formula to Tsusaka.
Since his depetrification, Senku was always wary of Tsukasa and his goals. Because he was too strong, Senku had no other choice but to consider the possibility of Tsukasa and him not sharing the same principals. Tsukasa however deeply respects Senku and asks him to abandon science so that he doesn't have to kill him. Tsukasa then shares his regrets of not having talked to Senku 3700 years ago, saying that he may have became his very first friend, to which Senku replies ''probably''.
Kohaku sees Senku as a gentleman and she is rather impressed by him. She first saw him sacrificing himself to save Yuzuriha and since that moment, she admitted that she was very interested in him. She led him to the Ishigami Village where Senku met all the other villagers. She did not trust in Senku at first but along the adventures, she accepts him and trusts him with very important things, like Ruri's life. It is hinted that she might have a crush on him, this is due to her constantly blushing when he is around.
At first, Senku and Chrome had a rivalry. Chrome proclaimed to be a genius sorcerer and Senku claimed to be a scientist. Chrome showed off his skill to impress Senku, Koharu, Kinrou and Ginrou but was completely overwhelmed when Senku reveiled that he knew all of his tricks. Nonetheless, Senku was impressed by all the knowledge that Chrome had learned all by himself. In a way, he reminded him of himself. After a small dual between the two, Chrome loses to Senku and joins his kingdom of science. They then team up to overcome the challenges towards creating the cure-all for Ruri.
Senku uses the blue jewel powder to track her. chapter 62