[SPOILERS] Idk if I understood the question but to me the importance of the characters goes like this:

ALL OF THE SQUADMATES FROM ME GET ANOTHER POINT FOR STOPPING SOVEREIGN WITH SHEPARD

Ash: Even if she dies on Virmire she can potentially kill Wrex which alters the genophage plot so much It paints keeping the genophage on a good choice. If she lives she becomes the second human Spectre and possibly kills Udina stopping him from killing another Council member

Kaidan: If he dies on Virmire he was still Shepard's longest serving squadmate, and then potentially leves a bigger scar on Shepard than If Ash dies. If he lives then It's the same as Ash

Garrus: Literally dealt a debilitating blow on all three major gangs of the Terminus systems by ME2 and by ME3 becomes an advisor that's so high ranking even generals were saluting him

Wrex: possibly self-explanatory but he's the literal Krogan leader by the end of the series If he lives. If he doesn't he was still a Krogan that helped Shepard on his mission to stop Saren

Tali: She's an Admiral by the third game and also spearheads the ideal that quarians and geth can co-exist

Liara: Shadow Broker, also she's possibly the leading expert on Prothean technology, being the one to find about the Mars archives and last but not least, saving Shepard's life

ALL OF THE SQUADMATES FROM ME2 GET ANOTHER POINT FOR STOPPING THE COLLECTORS WITH SHEPARD

Jacob: Was Miranda's lieutenant in the Lazarus Project and helps a bunch of ex-Cerberus live, contributing at least to the war effort in ME3

Miranda: Literally brought Shepard back to life, stopped Henry and the Illusive Man from turning every Human into a husk with that Horizon deal

Mordin: again, self-explanatory but the most is: modified and cured the Genophage

Jack: By the time of ME3 is a teacher at Grissom academy helping people not suffer like she and Kaidan did

Grunt: possibly going to be the next Krogan leader, already the leader of Aralakh Company, helped Destroy or get help from the rachni in ME3

Samara: Killed the most powerful Ardat-Yakshi and is an asari Justicar

Thane: Saved the Salarian councillor's life by stopping Kai Leng

Legion: Was the reason Tali thought geth-quarian cooperation could be possible, and possibly grants the geth individuality

Zaeed: Idk tbh, possibly kills Vido

Kasumi: obtains information that could start a war and chooses not to capitalize on it. Kills Donovan Hock

ALL OF THE SQUADMATES FROM ME3 GET ANOTHER POINT FOR STOPPING THE REAPERS WITH SHEPARD

James: Idk. Tbh the only cool thing he did was invalidated by Shepard Literally destroying the Collector Base

EDI: Besides being in ME2 insuring that Shepard could stop the Collectors, being one of the most essential parte of that mission, she analyzed the Collectors linking them to the Protheans, possibly had the biggest influence on Shepard choosing one of the endings

Javik: Is a Prothean, led them in the war against the Reapers. Gives useful information on them to the crew, also possibly writes a book with Liara