Cyan is just an ordinary person, he works as a cashier at a convenience store. However, Cyan has a history with murders and assault, in fact, he worked as a mercenary in the past. Cyan appears to be mentally sound, while underneath, he is a truly broken man. Cyan rarely cares about anything, however, he has a son whom he is a great father to. Cyan should sound quite dead inside, as he has seen many traumatic things within his life. A deep voice is what we are looking for.
Most of Cyan's deliveries are deadpan, but he still has moments of strong emotion.

Examples of what we are looking for would be Kishibe from Chainsaw Man, Jotaro Kujo from JoJo, and Iwai from Persona 5.
Audio example: https://www.youtube.com/watch?v=1KY0vShpUS4