The two have known each other since childhood and Taiju supported his endeavors. Senku and Taiju are best friends. Senku is highly loyal to Taiju and vice versa, which is shown multiple times throughout the story. Taiju trusts Senku's judgement and doesn't question or doubt his decisions, which is shown when he quickly realizes something going on between Tsukasa and Senku and doesn't argue his own opinion of Tsukasa not being a threat. This is proven again when in a conflicting, life-threatening situation, he puts his trust in Senku to make the most appropriate choice - that being whether to risk opening a smoke signal for Tsukasa or gain help by meeting another living human other than the former.
Taiju was the first person that Senku revived and he later becomes a major part of Senku's plans to expand the Kingdom of Science, his strength and endurance proving vital to the Power Team. His strong bond is evidenced by Taiju's willingness to sacrifice himself to a pack of lions to save Senku's life and again compromise his well-being in order to protect Senku and Yuzuriha, and Senku not being able to hide the petrified Yuzuriha in order to prevent Tsukasa from using a hostage; not bringing himself up to prevent his friend's happiness.
While the two are pretty much polar opposites in pretty much every way, they are very similar in that they are both very resilient in the face of adversity. Taiju has an unconditional trust in Senku, despite not being able to understand the majority of his friend's plans, but manages to contribute with bouts of insight and his enormous strength of will.
Taiju has a massive crush on Yuzuriha.
Prior to the petrification, he intended to confess to her but was unable because of turning to stone. However, his love for her allowed him to stay awake for thousands of years. She is his first choice on who to depetrify. He cries tears of joy when he is reunited with her and apologizes for having her wait over 3,700 years.
Just as with his best friend, Taiju is willing to risk his life in order to protect Yuzuriha.