As you complete various quests, you can encounter some characters that can be "recruited" to the labyrinth. When a character appears there, you can unlock a passive bonus thanks to them. It makes the game easier. Simply talk to such character and create a connection - this will provide you with additional bonus.
Seria can be brought to The Calm by completing the quest The Sorrow's Prey in Circus Minor (M3.9). If you solve the riddle of the woman's death and awaken her in the Labyrinth, you will receive a passive ability Seria's Peace. It will increase Might, Speed and Intellect by 1.
Recruiting Nychtemeron is not an easy task. To add him to The Calm, you'll need to complete Beleazar and the Beast (one of the side quests found in Sagus Cliffs (M3.6). The quest requires you to decide how you want to proceed - choose the third solution i.e. freeing the creature during the night (after a previous conversation with Nychtemeron - check Solution III in the section regarding the said quest to learn more). If you were successful, the being will inhabit The Calm. Talking to him unlocks Nychtemeron Defense, a passive ability that grants a random fettle to any melee attacker.
Choi can become a part of The Calm through Severed Child, one of the side quests began in Valley of Dead Heroes (M12.14). To add her to The Calm, you'll need to complete the whole quest, including acquiring the Twisted Metal Merecaster, and using it to convince Inifere to close the The Endless Gate (M13.6). Once you enter the mere, use Persuasion or Anamnesis to convince Inifere to release Choi. If you've managed to do that, the girl will inhabit The Calm. Talk to her to unlock Choi's Balm, a passive ability granting +1 Training Level in Endurance.
The party will encounter Inifere during Endless Horror in the Valley of Dead Heroes (M13.6).. Regardless of whether you've defeated Inifere in combat, or convinced him to close the Endless Gate peacefully, the man will appear in your mind. If you activate the link, The Exile will receive a new trait Inifere's Madness - a passive trait giving you +1 Might Edge, + 15% bonus (one level) to Intimidation, and -1 to Intellect.
Small Nihilesh (M19) has a wrecked spaceship in its north-western part that constitutes for a separate location called Cantena Interior (M19.9). Enter the ship and start manipulating the ship's generator. The party will encounter the ghosts of the crew. Persuade them to enter your mind. The Exile will receive Cantena Intelligence - a passive ability that increases Max Effort of Intellect rolls by 1.