What is the Japense characters (Writing) on a black belt?

[Diamond Snake]

I think it is close as it can get right? His name is Kieran Marcus.

[shujika]

I'd have to have the Phonetic pronunciation of his name (hard to tell from the written form). "Katakana" is completely subjective (being a strictly "phonetic" interpretation of foreign words) if the individual is "happy" with how it sounds when the katakana is spoken "aloud", then that's good enough.