Figured since this was team building/assists, I’d help with the team building part rather than the assist parts, more focused on how you approach him as a team player than who goes well with him.
Because of how Nemesis is almost universally on point, there are a few ways to build with him that I have found. Nem/Damaging character/main assist, Nem/Assist 1/Assist 2, Nem/Main Assist/Anchor, Nem/BFFs. These are the merits of each without taking into account any other character. Note that since these teams are role based, you will find characters that fit multiple roles and can overlap(Such as Strider as an anchor, as well as main assist), but you already knew that, so we might as well begin.
Nem/Damaging Character/main assist. This is for those with a Nemesis who knows what he’s doing. The main goal with Nemesis here is to just get in all the time to get that DHC. Meter here is important more than the others. Because you will be DHCing as much as you can, having that Damaging character being decent on point isn’t a bad call. Of all the damaging people, the most damaging DHC I came across was the Super Skrull, clocking in at about 970,000+ with mashing. The main assist had better be a damn good one to, as he will be doing all the assisting for the rest of the team. Strider, for example, or Sentinel.
Nem/Assist/Assist is more about keeping Nemesis out as a dominant force than anything else, as one might guess. Each assist is likely helping Nemesis get in, helping Nemesis with a certain matchup or strategy, or both. Note that I did not use primary and secondary assists in a particular slot. Since both help Nemesis, I leave it up to you to decide the anchor. This is the most recommended if Nemesis is the key focus of the team, and your strongest character.
Nem/Main Assist/Anchor is almost always anchor dependent, or at the very least, Nemesis isn’t the main focus. This setup puts a lot of weight as one might expect on the anchor. Nemesis is usually here to be a meat shield and a kamikaze, taking as much damage as he can while dealing as much in return. Note that the Main assist, given his situation on being second, should be able to hold his own in combat, or make due with whatever assist the anchor has.
Nem/BFFs is just as it sounds: Nemesis is there to be a meat shield before the REAL team comes out, dealing as much damage as he can before once again dying(poor sap). Any two character combination will do, and while this is similar to Nemesis/Main Assist/Anchor, note that no assists is distinctly given to Nemesis in particular, and you are more focused on the other two. This is recommended if you believe your Nemesis is either so strong or so irrelevant you don’t want to dedicate assists to him(HA, not likely)
Based on these four groups, choose how you want Nemesis to work for you, and gauge how much stock you put into him to find which team setup works best for you.
Lastly, I’ve found that the more each of the 4 team types intercept, the greater the synergy. Here are a handful of characters that I find useful and how I view them(These are clearly only a few and my personal preferences. For a better list and rating of assists, visit OnlyWingedAngel’s video guide).
Doctor Doom literally goes anywhere on all 4 teams, Main assists, secondary assist, anchor, you name it. Both Plasma and Hidden Missles do the job well. Sphere Flame hits like a tank, as I’m sure someone has said by now.
Wesker is like Doom in that he can probably go anywhere, but his assist is significantly worse, only useful for unblockables and combo extending. Note that his DHCing is poor, but he has some potential
Strider’s assist and anchor potential are both strong, and he works REALLY well with the above two in addition to Nemesis. His DHC is piss poor, but you probably won’t want him 2nd anyways.