Lui is an energetic young boy living in Syrus village, there he is in charge of the watchtower. He hates minions, and is often a nuisance to both Levant and Mahbu, annoying them constantly. He is the one who gives Levant the Beetle Forest key, and accompanies him to the Forest Gates the first time.
There are many hints that lead to think that Lui is, indeed, the Masked Boy that fights Levant in Beetle Forest, such as both having the same pants, and the Masked Boy's intent to modulate his voice. This fact is supported by the existence of a track in the game's ost named Masked Man ~ Lui. Though Lui and the Masked Boy are most probably the same person, this leaves the question to how he entered the Beetle Forest, since he gave the key to Levant to open the gate. Most probably Koris helped him somehow, since the two are friends or at least know each other, as is revealed in Koris' shadow dialogue in the Beetle forest of the netherworld.