Sel has mentioned on the Discord that the only reason the names alliterate is simply because it's easier to remember them that way, so it's definitely not a part of the setting. Considering that there's no reason to assume he wouldn't break this self-imposed restriction when he thinks it's appropriate.
He could also be doing this on purpose because revealing a non-alliterative name later on would be deliberately clashing with this pattern in a way the player would pick up on, possibly to indicate that this name, or this person, is related to the real world rather than the alliterative names that are common in Kumon-Mi. That being the case this non-alliterative name popping up on a drawing related to a twisted and fragmented memory from the real world could be foreshadowing for this happening in the future.
