2978 - Lyle Norg is born the son of the top xenobiology researcher for the Science Police.
2989 - 11-year-old Lyle's research develops his groundbreaking invisibility serum.
2993 - 15-year-old Lyle realizes the implications of giving his serum to his father. He uses it on himself & joins the Legion of Super-Heroes in their second recruitment drive.
2995 - 17-year-old Lyle is elected leader of the Legion of Super-Heroes after Rokk Krinn steps down.
2996 - 18-year-old Lyle steps down as leader of the Legion of Super-Heroes, focusing on building the espionage team.
2997 - 19-year-old Lyle is given an ultimatum to release his invisibility serum to his father. He delivers a sample of his blood to the Science Police laced with a bio-synthetic trojan virus, giving Querl Dox access to the entire Science Police network. They are granted amnesty.
3000 - 22-year-old Lyle dies in the Legion of Super-Heroes engages in the Mage Wars when Mordu corrupts his flight ring in deep space.
At some point the Legion starts to fill up with team members that seem to only be there because no one had that power yet. Invisible Kid is absolutely on of those; as an outside observer he seems to exist entirely because invisibility is an obvious superpower to give someone, and because he happened to be invented during the height of the fashionability of headbands.
Admittedly, I hadn't every really given him much thought, but when you start to go back and read the comics you discover that Lyle actually brings quite a bit to the table. Yes, the Legion already features the single most intelligent scientist superhero ever concocted in Brainiac 5, but the fact that Lyle is a scientist first with a more applied, human perspective on his research actually give the team a voice it didn't have previously. Having someone invent a 'serum' that grants them a superpower that defies physics is a pretty old-fashioned origin, but in the world of the 30th century it actually totally works. Also, it isn't treated as a throwaway plot device; the people in the story all acknowledge how revolutionary his work is, and keeping it out of the wrong hands actually becomes a very cool story premise.
Meanwhile, you also have the fact that the character briefly lead the entire Legion before focusing on his own smaller branch group. If you're going to grant someone invisibility as a super power you better have them ninja-stealthing all over the place, and Lyle is actually a fantastic example of that. They've implemented his powers in all sorts of cool ways, and having a genius scientist as your infiltration and stealth specialist gives you all sorts of awesome story potential.
Also, it bears mentioning that over the many iterations of the Legion there really haven't been that many character that have actually died. They usually get brought back in one form or another, if not in their own version of the Legion than in another... but Lyle's death during the Mage Wars story line was actually pretty epic, and brings a real finality to the event that we're using to dissolve the Legion.
So far from being one of the characters that was thrown onto the Legion just to give them a higher membership count, Invisible Kid is an indispensable and unique addition to the team.