Chika and Shiba knew each other before Chika's accident. They went to the same jr. high and were good friends. For Shiba life was boring after Chika left, so boring in fact he committed suicide.
When they found each other again, Chika was happy to see his friend, but Shiba was secretly an illegal zombie and the assassin Chika and Shito were trying to find, known as Butterfly.
Needless to say Chika was quite disappointed and shocked when he found out the truth, but he still refused to let Shito shoot Shiba, getting injured himself to stop him.
It can be argued that Shiba likes or liked Chika, since he did lick him. But Chika only thought of Shiba as a friend. Until the end of their confrontation, Chika held to some hope of changing Shiba for the better, and saved him from falling off the rooftop of a building. However, Shiba refused to listen to Chika and let himself fall to be reaped by Zarame.
That's as far as the anime goes. Chika and Shiba were friends, they ended up places as enemies, but Chika still held some hope. The outcome is not known since Shiba is rumored to come back later in the manga and Zarame's missing core is quite suspicious.
Overall, Shiba had betrayed Chika's trust, but there's a chance Chika might still forgive him. However, if he keeps going against Z-Loan, Chika will reach his limit. As for the pairing possibility, I (Mikari) do not ship them. If there's anything going on it's one sided from Shiba.
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