It can be however you interpret it. For me it’s mostly dialogue.
Thanks, that helps define the question for me, as the most memorable game characters are based on dialogue, personality, and voice acting. But narrowing it down to 4 characters is tough for me, especially for an avid player of Bioware games who have created some of the most memorable characters I've experienced.
The first 2 are set in stone for me:
Minsc (BG1 &2) - by far the most memorable, eccentric, "whacked", and unique video game character I've ever experienced. Minsc, voiced by Jim Cummings, has a memorable voice, and some of the most iconic lines in PC game history.
View: https://youtu.be/X8lLruVR2zQ
Garrus Vakarian (ME1-3) - the ultimate "I've got your back" companions, voiced by Brandon Keener. He'll stick with you through all 3 games, no matter your choices (not to mention he's great with a sniper rifle). The iconic line, "I'm in the middle of some calibrations" is joked about in ME3.
View: https://youtu.be/4pkNhA8hf_s
Spots 3 & 4 are tougher, because there are many characters that could fill those spots, so I'll list 2, though they're not necessarily exclusive:
Morrigan (Dragon Age 1 & 3) - Morrigan, voiced by Claudia Black, has a complex personality that is often brutal, sarcastic, but can also be a bit sensitive if you dig far enough in the conversation options. Good luck trying to romance her in DA Origins.
View: https://youtu.be/dI_OxWGB9vM
Oghren (Dragon Age Origins) - voiced by Steve Blum, is one of the unheralded characters from the DA games. More of a tie between him & Alistair (voiced by Steve Valentine), as they both have some really funny dialogue.
View: https://youtu.be/uwpMlOQe_X4
As a side note, I have to say that the importance and quality of voice acting in PC games can't be understated. It's the quality and feeling that's provided by voice actors and the voice actors guild that really gives depth and feeling to video game characters and those memorable characters we experience wouldn't exist otherwise.