Ryko is the Moon Shadow that guards Lady Dela at all times.
Before being "placed in the palace", as he puts it, his life was one of thieving, fighting, and drinking. However, he relates that one night two men equated him; according to him, one of them knifed him on the shoulder, while the other knifed his belly, to the point of him seeing his own guts. A fisherman saved him, and, to pay his debt, Ryko joined this man's cause; the Islander Resistance. He ended up at the palace because this fisherman needed someone to infiltrate the palace, and he did so to repay his debt.
It is mentioned that he can pick locks, and it is proven truth when he helps Eon enter Lord Ido's chambers in search for the lost scroll. He is a very skilled fighter, and kills without any regret.
Ryko is an extremely loyal person, and is very obedient in his duties to guard Lady Dela. When Ryko finds out that Eon is a girl, he is very angry and doesn't treat Eon with as much respect, but he still does his job and serves Eon in order to help the resistance. Near the end of the book, Ido tortures Ryko by stabbing his hand in order to get Eon to stop attacking him. Although he is in a lot of pain, Eon knows that Ryko wouldn't want her to drop her swords, and he even whispers "Don't" at her, but she drops her swords anyway because she can't bear to see him in pain.
Ryko and Eon have no romantic relationship, but they are very loyal to each other. It is stated that Lady Dela is in love with Ryko, but Ryko does not explicity say he loves her. Although it is briefly suggested, there are no romantic relations between the two in the first book.
Ryko is described as being extremely strong and powerful-looking several times throughout the book.