Verse: Verse is a Scarlet Fury, and will typically be a damage dealer in your party. She also tends to be fairly squishy, which limits her usefulness over time. You automatically recruit her early on. In fact, she is the third allied character you encounter in the game, in the very first area. She is a solid DPSer on her own, but paired with Barik, she is even better, as the two of them get their own special combo attack that is very effective. Consider using her if you plan on using Barik and your main character doesn’t fit the DPS or Tank role.
Barik: Barik is a walking tank. In any playthrough, he is likely on your list of potential meat shields. Barik is granted to you after initially speaking with the archons in the Disfavored camp. Of all your companions, he is also easiest to lose due to plot decisions – be sure to build up at least a loyalty of 3 with him before the end of Act 1, or he may not side with you over Graven Ashe. Obviously, this is not an issue during a Disfavored run.
As a note, if you plan on using Barik, construct a Forge for your first Spire upgrade. Barik has some of the best armor in the game, but it starts at Common quality, and cannot be unequipped. If you give him some love at the forge however, his armor can be upgraded to be better than any other armor set in the game. Crucial for him to remain relevant in the later stages of the game, especially on higher difficulties.
Lantry: Lantry joins you after you save him from his Scarlet Chorus captors. He joins automatically, and is easily the best magical character in the game, with good options for magical specialization, a uniquely high Lore skill for better spells, and the ability to use his quill skill tree to be competent with his regular attacks as well. I always auto-include him in any party of mine. The only way to learn the Sigil of Life is for Lantry to teach it to you.
Eb: Eb is… a more offensive mage than Lantry. She is fantastic at this role, but I personally did not use her much, as my main character almost always filled her role to some degree. That said, she is very good at filling either a support or offensive role. What’s more, her magic saps health, and her skill tree allows her to do more with Terratus spells than most. She is recruited automatically after acquiring your first spire. The only way to learn the Sigil of Terratus is for Eb to teach it to you.
Sirin: Sirin is a bit of an odd duck. She has skills that keep enemies from targeting her, and her role is almost exclusively support magic. She can also do some semi-effective debuff magic for enemies, but I find that skill tree unimpressive. Unforunately, Lantry is a better all around mage option. Sirin is fun to use, though, and worth recruiting. She is in Lethian’s Crossing, in Sirin’s Sanctuary, and can be recruited immediately in Act 2. Know that if you put off recruiting her long enough, she may disappear, so don’t dally too long. If Lethian’s Crossing’s main questline gets skipped, Sirin leaves, so recruit her first.
Kills-In-Shadow: Kills-In-Shadow is hilarious. She is a beastwoman, uses no equipment aside from trinkets, and does tons of unarmed damage. Her skill trees both revolve around different types of damage dealing. She can fill either a DPS or tank/control role, and I have used her to great effect in either. She will eventually interrupt your travels in Act 2, at which point she can be recruited.