I'm Luneth! Nice to meetcha!
One of the four Warriors of Light. A reckless boy who explores the Altar Cave shortly after an earthquake. There, he fell into a hole and discovered monsters in the cave. In the depths, he found the Wind Crystal, which dubbed him a Warrior of the Light and urged him to gather others.
Luneth is the de facto main protagonist of Final Fantasy III: He's the first main character you take control of, he is the closest in demeanor to the original Onion Knights and he clearly seems to be inspired by the white haired Warrior on the cover art.
Luneth's personal sigil is based on his vest and the three belts that make it up. This usually takes the appearance of a 3x3 grid.
Just looking at the final game without looking too closely, Luneth appears to be just a straightforward Final Fantay protagonist. He has shades of personality, and some can pick up on how he's intended to be read, but there's generally not much remarkable.
Factoring the unused text? Well...
If one were to describe Luneth, words like "reckless" or "careless" come to mind. Luneth isn't the quite sort of bratty protagonist you'd expect from this sort of game (especially given a certain someone in Dissidia). Rather, Luneth doesn't seem much for courtesy and lacks manners or any degree of tact. This is more clear in the Japanese version where he uses the Japanese pronoun ore, which is generally seen as a casual way of addressing yourself, even in front of Princess Sara (though Sara doesn't mind, which makes some sense given her nature). You can still kinda see it in the english version. Overall, Luneth generally keeps a light attitude and is the type to joke around or otherwise be carefree.
Of course, it isn't all sunshine and rainbows for Luneth. He can be serious on occasions, and is occasionally made to be introspective. However, Luneth seems to enjoy cultivating an upbeat, somewhat boisterous image, with him brushing off the feeling of Deja Vu he feels on the Wrecked Ship and Refia noticing whenever Luneth is taking a situation seriously. On a related note, Luneth is rather proud and likes to make himself look tough although, unlike a certain Onion, he generally tries to back that up even prior to his character development (on the contrary, Luneth seems to suffer from the opposite problem).
Still, Luneth does have his moments of compassion. He gives a surprisingly serious declaration to protect the people of the Floating Continent in the unused epilogue for Castle Hein, one that even Refia is surprised by. His interactions with Aria, already noticable in the final, are even more noteworthy in the unused text as he clearly shows concern and care for her that was much more rare elsewhere. And, of course, some of his interactions with Doga and Unei are reflective, culminating in being the one to vocally hesitate when Doga and Unei fight the party.
Overall, Luneth leans more in the direction of characters like Zidane: He's a good-natured boy at the end of the day, but he has flaws. Flaws that occasionally cause friction with his party, mind you, but he's still the leader at the end of the day and he does a fine job of leading the party.
It's just a shame that most of the deeper aspects of Luneth's character got left on the cutting room floor.
Luneth is one of the most consistently portrayed in terms of using his 'canon' job.
Warrior is Luneth's 'go-to' job and the one he is most consistently portrayed as. One could argue that it's due to the jobs "Advance" command: Someone as brash as Luneth would likely probably subscribe to the idea that a good defense is a strong offense.
For potentially similar reasons as the Warrior, Luneth is occasionally portrayed as a Dark Knight; He has a Dark Knight unit in Brave Exvius and access to some minor Dark Knight skills in Record Keeper.
This is largely due to Final Fantasy Record Keeper, which heavily portrays Luneth as a Dragoon, helping cement him as a wind unit. One could argue that this is inspired by the moment in the CG opening where he leaps at the Behemoth.
Probably the biggest storyline that concerns Luneth revolves around Castle Hein. Unlike in the final, the initial plan was for Luneth to apparently be seperated from the rest of the party. He would meet with a man named "Falb" whom the party had encountered earlier in Tokkul. Falb's family had been killed by Hein and the man wants revenge. Thus, he had previously set out to find "Emblem of Bravey" (勇者の紋章): A crystal piece that grants the user the Evoker job, one of the few that can sidestep Hein's Barrier Shift ability. Despite Luneth being impressed, Falb feels that he's fighting his own fight and it's something he has to do by himself.
Falb soon stages a jailbreak and holds back the guards, allowing Luneth to hunt down his crew. By the time the party comes back, however, Falb is too weak to join them. He passes the crystal piece to Luneth, granting him the Evoker job before asking him to defeat Hein.
Falb appears later in Castle Argus. With Hein defeated, Falb thanks Luneth for his help and devotes himself to rebuilding Tokkul.
While rescuing the party, Luneth has a brief talk with Ingus, the latter pointing out that Luneth normally would be too prideful to go out of his way for help. Luneth admits that he is conceited and that he was too overconfident before wondering if he has reached his limits.
Aria is probably Luneth's most interesting interaction with one of the guests. While the final game does carry implications, the unused text makes Luneth's feelings for Aria a bit more apparent; Luneth shows surprising concern for Aria that he doesn't really show outside of Doga and Unei's final test. Refia even speculates that Luneth forgretting his seasickness is partially due to Aria.
Of course, there's a bit of a contrast. Luneth is very easygoing while Aria is driven in her mission to restore the surface back to its original state. This is touched upon in one of Aria's unused talks, which itself is followed up on in another skit.
The interview in the strategy guide confirms that Luneth was in love with Aria.
In Final Fantasy Record Keeper, certain characters can get interactions with others in the "X Records" event. For Luneth, he gets an interaction with Bartz and Zidane, both of whom were also upbeat protagonists (with a mischevious side in the case of the latter).
Luneth's second dialogue, interestingly, seems to come from an unused dialogue for after the party defeated Gutsco.
As the representative of III in spinoffs, Luneth and Onion Knight don't have any in-story connection. Luneth's early appearances in mobile games such as Record Keeper and Brave Exvius gave Luneth some of OK's moves (in particular, RK had a Dissidia event and had to give Luneth syngery since OK wasn't in the game yet). For the most part, however, they are treated as seperate characters, to the point that Onion Knight's real name is kept ambiguous.
As of this writing, the only time Luneth and Onion Knight have exchanged dialogue is during a Nemesis interaction in Record Keeper.
Onion Knight does have a Luneth-inspired costume; The costume gives him Luneth's facial features including his hair and his eyes, but combines it with the artwork character by making the armor blue. Further removing the possibility of both being the same character is that the costume is called "Luneth-Style" (ルーネススタイル) in Japan. This is not to be confused with Onion Knight's "Bladewielder" outfit, which makes him more closely resemble the figure from the Amano artwork for FF3.