Talk:Kuroyuri (episode)/@comment-58.170.146.216-20170122200551/@comment-25936766-20170122203632

From what I know of Japanese Culture, calling someone by their first name tends to be seen in a more personal light, used generally for close-close relationships....But that explanation works one-sidedly here.

Personally, the simplest answer is just that his friends prefer to call him Ren and Ren either prefers it or doesn't mind. It's not strange at all, it just happens. My family calls me by my first name all the time, yet my friends call me by my family name all the time, and I don't mind either way.