A lot of Pokemon's main cast are some of the most memorable video game characters for thousands if not millions of players. Some reach such an iconic legend status that they'll be talked about for decades (Professor Oak, Giovanni, Cynthia, ...)

There are some that unfortunately won't enjoy this fate. I'd even go as far to say that most players won't even remember their name by the time they enter the next area. Trainers who might honestly be lower on the memory totem pole than Youngster Joey, or that guy in Brock's gym.

The first example I always think of when this topic comes up (which ironically might mean they don't fit the description after all... what a paradox), is Will.

Who's Will? Yes, I just read your mind. Will is the first Elite Four member you face in the Generation 2 games (and their remakes). He is a Psychic type expert. He has a mask? Honestly that's all I can really tell you.

The problem with the Elite Four in general is that they are almost always so excluded from the story that you really don't end up caring much about most of them, despite their often creative designs. You usually meet the champion throughout the game, and have a big interest in their character/battle as a result, either through anticipation or the surprise factor. The Elite Four almost just feels like stepping stones on the way there.

There are good one-off Elite Four members that do make a big impact without much lore relevance. Drake from Hoenn is a good example of this. He's got a great design, exudes this threatening aura, and has a strong team to back it up. Then there are others who fail at producing that lasting impact on the player. One I'm always disappointed by is Kahili in the Alola games. Really fun design and theme, unique type for an Elite Four member, only to be very forgettable in the end.

As the Elite Four fights of the Johto games also take place at the Indigo Plateau, there are some similarities with the Kanto Elite Four, such as Bruno and Lance still being there (with both getting promoted up a rank). The other Kanto Elite Four members got replaced by new ones. Will takes Lorelei's spot as the first. He has pretty generic dialogue about training with his Psychic Pokemon and being a new Elite Four member.

In battle he does have a fairly unique team. He actually keeps two of Lorelei's Pokemon, a Slowbro and a Jynx. He then adds an Exeggutor for a very Kanto-heavy team. Seriously, why not let him have the new Slowking? Anyway, he does have two more Johto Pokemon, including his ace. Only... they are both Xatu's. It's not unheard of for Elite Four members to have repeats, but still a bit disappointing. It's a cool Pokemon at least! He keeps the exact same team in the remakes, by the way. At least when you rematch him he ditches Exeggutor and one of the Xatu's for a Bronzong, Grumpig, and Gardevoir. Those are some unique choices I suppose.

Honestly that's about all I can say about Will. He is never seen or heard from since, outside of spin-off appearances in Stadium 2 and Pokemon Masters EX. Probably the single most forgettable and irrelevant 'Important Trainer' in all of Pokemon