Avy

Overview
Unique Succubus. Avy can be recruited from a Town portal event. The event simply consists of choosing whether she joins or not, and you can pick a type which determines the Level Type and Skills of Avy and her pet NightCrawler. By default the choices are Magic, Dexterity, Seduction and Tamer, with the additional option of Jumper being unlocked after reaching her world.

See Night Crawler for Familiar's skills.

You can also choose to recruit Alice instead.

Combat
In terms of Stats, Avy's only weakness is a lack of Lust Resistance and average Stat Growths. Naturally, her chosen type will influence her combat style as it changes up her Level Type and Skills. Regardless of your choice however, Avy has the unique passive Instability, which will change the first skill, FireBall by default, into another spell at the start of every turn. Since it changes at random it could either be a life-saver or useless.

If you wish to utilize this quirky style Magic is the obvious pick. As the skill is selected from a roster of magic spells, a Magic Level Type is just what you need. Reroll will also allow you to invoke Instability once a turn, free of charge, should you roll a dud. Even if you fail to end up with anything useful, you'll have the powerful EnergyBomb to fall back on.

If you don't want to gamble your chances of victory on getting the right spell however, there's numerous other choices that will let you disregard that first skill. Dexterity will give Avy beastly DPS potential, while Seduction gives her the skillset required of a Succubus.

Tamer shifts the focus from Avy herself to her pet Familiar, with Tail whip and Focus helping it evolve, as well as any other Creatures that you might want to tame. This is the one you really want if you wish to proceed with Avy's questline. That will unlock Jumper for future runs, which gives you a shortcut to evolving Familiar, while giving Avy battlefield manipulation capabilities.

Questline
As of V16 she has a quest attached. Wanting to know why she has suddenly been left with the MC, she decides to summon her friend (Xhana recruitment event). After hearing what she has to say, she decides to evolve her NightCrawler in order to go back. Avy's NightCrawler has a unique skill that allows it to eat an enemy to increase max mana. Once it gains 100 max mana it can evolve into a PortalRipper, with the ability to create a stable portal to Avy's homeworld. Upon entering this unique world, the MC learns some recipes involving materials that can only be acquired in this world. Upon reaching the city you get the choice of saving Avy's teacher (recruit Ryxhora, the succubus that left Avy with you), fighting to free this world, or to simply leave. Winning the fight presents more choices.

Trivia

 * Avy's questline is the most complex one with a range of different outcomes depending on your choices, when it doesn't even involve an ending.