The following is the memorial plaque wall when you start with a new character and select the "numerous" option. As you'll note, most of the squad members from Mass Effect 2 are still alive:
Those names underlined in blue are the original crew members of the Normandy SR-1 who died in the beginning of Mass Effect 2, along with the two squad members Ashley Williams and Urdnot Wrex (marked by blue squares). Kaidan was chosen to be alive because I was playing a female Shepard; when playing a male Shepard, Ashley will be the Vermire survivour.
Those names underlined in orange are all crew and squad members from the Normandy SR-2. Those marked with an orange circle, Gabriella Daniels, Kenneth Donnelly, and Kelly Chambers, would have all made notable appearances in Mass Effect 3 had they not died. More specifically, the former two can be brought back aboard the Normandy in their usual positions as engineers, and Kelly Chambers is a potential romance/fish keeper* depending on your interactions with her in the previous game.
Thane Krios and Jacqueline Nought (Jack) appear to be the only squad member from Mass Effect 2 that's listed as having died, and are highlighted on the wall by the orange square.
Everyone else not on this list is presumed to be alive, and not one of the "numerous who died." In particular, Garrus, Tali, Miranda, and Jacob are all listed as alive. However, the game will act as though you never met Samara/Mordinth, Grunt, Katsumi, and Zaeed, effectively killing them. It will also act as though the original Council died during the attack on the Citadel in Mass Effect 1.
*that is, she returns your fish from Mass Effect 2 to you; you need the aquarium VI or extreme patience to keep them alive
The problem of course is that this is the same list of people who would have died if you had simply chosen the Ashley/Kaidan option. In comparing the two save files, one from a "numerous" selection, and one from a "Kaidan" selection (using male Shepard to keep a consistent Vermire casualty), there's actually very few differences between the two scenarios.
The tables below illustrate the difference in game variables before you perform any action:
PlotID | Numerous | Kaidan dies
--------+----------+------------
17807 | true | false
18005 | false | true
18122 | false | true
21569 | false | true
PlotInt | Numerous | Kaidan dies
--------+----------+------------
10393 | (unset) | 3
At the moment it's unfortunately unclear what these changes represent, because they don't directly map to any of the significant and/or labeled plot events that the game has. Presumably they have some noticeable impact, however, so I'm going to keep digging until I get to the bottom of this.
Best Answer
The Illusive Man is the leader of Cerberus. He is a shadowy figure that few people meet in person. In Mass Effect 2, he "recruited" Shepard and gave him/her the tools needed to investigate the Collectors.
For instance, the version of the Normandy you control in ME3 is Cerberus-built at the Illusive Man's direction, and Shepard has certain implants that were manufactured by Cerberus as well. Some of the people you'll meet in ME3 are active/former employees of the Illusive Man. Miranda in particular was a key Cerberus operative/Illusive Man informant during the events of ME2.
However, he and Shepard parted ways at the end of ME2. Depending on your actions in that game, he can be more or less angry by the conclusion of ME2, but it seems that no matter what you do, the Illusive Man and Cerberus will be your adversary in ME3.
You could classify him as an "evil genius" - he's got good ideas and wants to ensure the survival of humanity, although his methods are ethically questionable. He doesn't trust other races, and is actively hostile towards them at times. As Shepard's goal is to unify the Council races against the Reapers, he/she is frequently at odds with the Illusive Man's methodology and tactics.
His tendencies to encourage brutal experimentation on living creatures, his desire to control, engineer and evolve humanity into the dominant race in the galaxy, and his rampant xenophobia draw strong parallels to other historical tyrants. It's arguable that he represents as real a threat to the galaxy as the Reapers do.
Typically he is seen sitting in a chair in a large room, surrounded by holographic displays. He frequently smokes, and has bright blue eyes. His appearance is modeled after Martin Sheen, who provides his voice. His backstory is left mysterious in the games, but has been expanded upon in other literature. The comic series Mass Effect: Evolution contains a good portion of his backstory.
Also, don't confuse "Illusive" with "Elusive" - it's arguable he's both, but I think many people think of the latter term when they think of his name.