My father has changed... It is as if some evil spirit has taken ahold of him.
Alus Restor is the Prince of Saronia, the singular kingdom on the surface. At the time of the game, the kingdom is plunged into a civil war by the king, and when Alus tries to intervene, the king banishes him. Eventually, Alus ends up in a tavern where is bullied by a group of thugs who don't recgonize him. Luckily, the Warriors of the Light arrive and beat them back. The Warriors team up with Alus to get to the bottom of the civil war. But when they head for Castle Saronia, it turns out that the king is suddenly willing to let Alus back in.
Yup. It's a trap.
In the dead of the night, King Gorn approaches his son with a knife and nearly stabs him. However, Alus crying out causes the king to come to his senses for a moment and stab himself. As it turns out, the king was being controlled by the threacherous Gigameth/Gigametz, who in actuality is the fabled winged fiend of Saronia: Garuda. The Warriors of the Light succeed in defeating him, but the damage is done: Gorn reaffirms his love for his son and dies, and Alus ascends the throne as king of Saronia.
Of the various party members, Alus kinda draws the short end of the stick. The Saronia chapter is potentially the shortest chapter in the game, meaning that he won't be in your party for long, the nature of the chapter means that it is impossible to take him outside of Saronia (mirroring how Sara cannot leave the Inner Sasune area due to Nelv Valley being blocked) and his big scene is reactive compared to the other guests who are more proactive. Then again, Alus is, like, 10 and he lacks most of the shenanigans that other such youngers in the series possess to allow them to lift above their weight class, so that's understandable. Still, his track record even extends to spinoffs, as his presence is non-existent compared to all of the other guests, even including Sara and Cid.
Personalitywise Alus is primarily in a sort of lost state; questioning what is happening to his father and why he has been banished. He is implied to be perceptive and smarter than his age would suggest though: When meeting with a member of the castle staff whom recognizes him, Alus tells the man to be quiet, as he's exhiled and must keep a low profile. Additionally, he ends up becoming king of Saronia, with the implied duties that that entails.
The fact that his aides give him Dragoon equipment would imply that he could theoretically use it. However, there's no evidence of him doing so and the remake goes in a different direction with his movelist.
The Final Fantasy III remake expands on Alus in a number of ways. His implied wiseness is more apparant here as he acts as a guide to Saronia for the Warriors of Light, expositing on the function of each of the four districts and reassuring Refia that he's too young to drink when she questions why he was in the tavern.
Most infamously, Alus has a deep bond with Arc in the remake. Having grown up being bullied himself, Arc empathizes with Alus' initial situation and strives to aid Alus in his predicament. One conversation even has the party watch Arc as he talks with Alus and wondering why he's so eager to help. Arc inherits most of the interactions the WoLs had with Alus in the original, up to being the one to sleep in the bed next to his (in the original, all four kids were implied to be in the bed; In the remake, Luneth, Refia and Ingus are implied to be in another room and run in when Gigameth arrives).
Like the other guests, Alus aids in battle randomly. Much like Sara, Aria and Unei, Alus is a White Mage-esque character, using Aero and Confuse as his moves.
Alus is expanded upon a bit further in the unused text. He's shown to be knowledgeable about Saronia's history, able to explain to the group the backstory of Saronia and how the Dragon Spire came to be. When Arc leaves the party to find the key to the Spire himself, Alus feels guilty and accompanies him to make sure he remains ok. The plot point of Alus hiding his identity actually does come into play here; Arc pretends to be Alus when talking to one of the aides and Refia mentions that Alus having to spot Arc when they try to purchase a spear would blow his cover.
Alus' denounement at the end of the Saronia chapter is also expanded upon and a bit of nuance appears that seemingly got dropped in the final: When referring to the Warriors of Light, Alus uses additives that mean "Mr." (for Ingus) and "Ms." for Refia. By the end of the Saronia arc, Alus does not use "Mr." for Arc, instead simply referring to him just as "Arc". So while Arc isn't offered a position like Ingus (Minister) or Refia (Nanny, much to her restrained horror), Arc is content with his bond with Alus.
The Final Fantasy III manga also features Alus, but in a wildly different role from the games.
After the Warriors of the Wind arrive on the surface world, they soon find themselves surrounded by monsters. The monsters are beaten back, however, by Alus descending from on high with a gaggle of instrument-wielding sages.
Yes, really.
In this continuity, Alus is the prince of Solrados, a kingdom that was on the surface. Unfortunately, the great earthquake affected the surface too and the kingdom was destroyed. Alus is seemingly the only survivor and the poor kid had to watch his mom, his dad and his sister get washed away because Alus is not allowed to have family in any universe.
Alus had a pretty awful life in the manga.
Alus accompanies the party for a while, but they soon find themselves at the mercy of a massive earthquake. Alus' entourage of musicians/holy men/??? spin really fast in a circle, sucking up water and unearthing the crystals. Unei is released and restored to her youth, but Alus was tied to her as a "dream child" and was already dead (I think?). With Unei restored, Alus dies and Unei sends him on.
Overall, Alus is an interesting, if bizarre, combination of his game counterpart and Aria. Not sure why the manga went this route, but it sure is something.