If you ever feel like your team is missing “something”, these characters can typically fill in that something. If you put together a two character combination that’s a little “weird”, one of these characters will come in with the love and support and make that combo stronger. Each character’s move set holds something that can fulfill a variety of roles. Lockdown, Space-Control, Defense, Combo Conversions, and even a Marvel beam assist from Robo Fortune. You’ll find one of these characters on a large majority of teams, as they have some of the strongest assists, can fit into any position on a team, AND don't require specific assists to bring out their potential, bringing lots of flexibility in both choosing partners, and changing your order and assists on a match-to-match basis.
That said, you don’t NEED to play these characters. Several top level players don’t, you’ll just have to think a bit harder if you want to “optimize” your team composition.
*Robo Fortune does not have the same team position flexibility as the other Glue characters as she is not the most ideal anchor character due to her comeback potential being pretty paltry with her low damage and poor defense. That said, her Theonite Beam assist easily being one of the best in the game means that it's never a bad idea to throw her on a team, even if it's as an anchor.
Every character makes for a good point character since everyone likes getting extra help. The points mentioned here just have something special worth noting.
The Selfish characters have assist/DHC support that is overall poor in most (not all) team compositions. They typically bring more value to a team by being enabled by their partners to go wild.
The Battery characters build a lot of meter while also not relying on meter to optimize their gameplan. Peacock, for example, builds most of her meter through her zoning patterns, and while she can spend this meter to strengthen her control of neutral and capitalize more from her zoning, she doesn't NEED to do so to remain a high level threat. Aside from the selfish characters (for most team compositions), these characters can usually fill other slots on a team pretty well.
These characters, as the name says, use resources to reach their full potential. These resources are built as a part of their own moveset, and not to be confused with super meter. Said resources can be risky to build solo during neutral play, so they all greatly benefit from assists that give them time to build resources in the middle of a combo, while also letting them continue the combo afterward. The most notable of these types of “Resource Assists” are:
Umbrella [Salt Grinder] - Keeps the opponent in hitstun long enough for Valentine to load 2 Vials, Robo to summon 2 Heads, Squigly to charge both stances at once, and lets Beowulf get a Taunt out of Grab Stance. Also leaves the opponent in a stagger state, so characters with command grabs can combo into them. On TOP of that, it also does pretty good damage. The price for all of this, however, is that it is absolutely useless in neutral.
Cerebella [Excellabella] - Amazing for Valentine and Robo Fortune. One of the few (good) assists that lets Val build two vials in a combo.
Big Band [H Take the ‘A’ Train] - Beowulf's favorite assist, letting him gain full Hype in a combo from nearly any situation while also giving him the game's highest meterless damage output. Also fantastic for Robo Fortune.
Fukua [M Forever a Clone] - Great choice for Beowulf, Robo Fortune, and Squigly, generally good combo and assist for the rest of the cast while also being good for neutral play.
Valentine [Mortuary Drop] - Requires that your point character is able to inflict a stagger state on the opponent to combo in to (which all of the above can do easily), but goes through a very lengthy animation allowing Beowulf to get full Hype, Squigly to charge both stances, Robo to summon 2 heads, and Valentine... unfortunately is the assist character so she cannot build vials.
Outside of these assists that generally get used in the middle of a combo, good neutral assists that inflict a lot of blockstun or otherwise keep the opponent off of you are also good for creating time to build resources. These assists will be mentioned on the character pages as you come across them.
You’ll often see these characters on point, sometimes mid. Usually in trios to have a resource assist and another assist to assist in neutral/defense/pressure. Generally fine to pair two of them on a team together, as while they are resource characters, it's not like they are fighting over the same resource.
Characters that were too cool to fall into any other category. To go a bit more in-depth:
Filia, Fukua, Parasoul, Squigly, and Umbrella can mostly go anywhere on a team and have good assists, but they aren't on the same level of utility as The Glue characters.
Painwheel's assists are either really good or really bad depending on the partner, but she has one of the game's best DHC options with Hatred Install.