He seems to possess powers that have gone as-of-yet unnoticed by Guts and his friends, but Schierke has noted that the boy's Od is different from that of a normal human.
The Moonlight Boy is a young child whose most notable features are his long black hair and wide, knowing eyes. On the two occasions that he joins Guts' party, he is naked upon first encountering them. Though his age is unknown, it can be deduced as younger than that of Isidro and Schierke, judging by their differences in height.
By virtue of never speaking, the Moonlight Boy's personality is largely unknown. What can be observed, however, is that he and Casca quickly form a close bond upon meeting, and his relationship with her remains the strongest out of the rest of Guts' party, akin to that of a mother and son. The Moonlight Boy appears to have a sense of curiosity and playfulness; he bashfully averts his gaze from Guts' after they lock eyes, but then proceeds to climb onto the swordsman's back when the latter isn't looking.
Despite appearing completely human, the Moonlight Boy seems to have some form of persuasive control over astral creatures. When crocodile Pishaca attack the cabin that Guts and his group are staying in for the night, the Moonlight Boy simply stares at the one entering the broken door. The Pishaca pauses, stares back, then withdraws. This saves the largely defenseless Farnese, Casca, Ivalera, and Schierke.
Guts suspects that the Moonlight Boy has the power to appear to him personally in the guise of a child-shaped creature made of flame. The creature can only be seen by Guts and has thus far appeared twice before him: once on the beach near Vritannis, where it prevented him from harming his friends under the influence of the Berserker Armor, and once inside the body of the Sea God, where it helped him escape before he could die of suffocation, hunger, or drowning in the Sea God's blood.
The Moonlight Boy also has the ability to interact with and "ride" the branches of the World Spiral Tree, something which only he has demonstrated the ability to do currently.
Casca, having wandered away from Farnese, initially encounters the Moonlight Boy along Vritannis' sea shore. The rest of Guts' traveling party find her gazing out to sea and holding the boy, having wrapped her clothes around him to protect him from the cold. When they approach, Schierke senses a mysterious Od emanating from the child, and the group decides to bring him back to their beach cabin.
While being taken care of by Casca, the Moonlight Boy appears drawn to Guts and sets about climbing his back. Casca immediately rushes to take him away from the swordsman, and the resulting scuffle nearly causes the child to fall on Guts' throwing knives. Together, Casca and Guts catch the Moonlight Boy. Isidro and Puck note that the three seem like a family, though Casca, agitated, quickly withdraws from her close proximity to Guts with the child in tow.
That night, Casca and the Moonlight Boy sleep like mother and child, the sight prompting Guts to remember Casca's instinctual bond with the Demon Child. He recalls his last encounter with the imp at the Tower of Conviction, and suspects that the malformed entity is still out there alone in the night.
- It is speculated that the Moonlight Boy and the Demon Child are the same entity. This would make the Moonlight Boy the human version of Guts and Casca's son, explaining the Moonlight Boy's deep bond with the latter and why he appears to Guts as the flame creature.
- Both entities display a strong desire to protect their parents, and both seem to have the power necessary to do so. The Moonlight boy bears an immense physical resemblance to Casca, and to a lesser extent, Guts. However, anything more is speculation, as the Demon Child was used by the Egg-Shaped Apostle to reincarnate Griffith.
- Schierke suspects that he is either an emissary from Skellig or else Sovereign Hanafubuku's avatar.