Enjyu's surname is now confirmed. In the original tweet, Inuki said that a fan had suggested "Kounoki Enjyu" for his full name and he liked it but he wasn't sure if he could use it. The update states that he got in contact with the fan and has received permission to use it, so Enjyu's official surname is Kounoki.
Please note all of Inuki's doujin are all set in the manga-verse of Battle B-daman so it is possible for this doujin to reveal a backstory for Enjyu that may contradict the anime.
Bonus preview:
(Note it's a fairly common occurrence for Inuki to use fan suggestions, a number of characters and details that were added to B-daman canon via his website were actually submissions of OCs from fans.)
For my second entry to the series, we’ll be touching upon everyone’s favourite mysterious demon-child, Biarce!
More specifically, his lovely little family.
The only boy in a surprisingly large family (compared with other characters of the series), Biarce almost sticks out like a sore thumb amongst his much less apparently evil kin.
Left: Arche* (アーチ) is bright and adorable. But she's weak-willed, and a complete worrywart over her big brother Biarce. Despite this, she's a very cheerful girl. She is right handed, and the twin of Sacha.
Right: Sacha* (サーシャ) is also cheerful and spirited. Nobody can beat her energy! She has her amusing goof-ups from time to time, but she's a good little sister who is kind to everyone. Her favourite thing is comedy. She is left handed, and the twin of Arche.
Top right: Alice*, (アリス) Biarce's 15 year old sister, is a little bit like him in that she's very cool. She's a good, caring girl but sometimes has a sharp tongue and snappy comebacks. She once was by Ababa's side with her brother, but left as Biarce was far better at what they were doing than she was. (She is good at divination, however). Since then, she now works as a referee for the IBA.