In your first session, you may discover a slot where you can specify a custom ability. It'll be a very small ability based off your Classpect thematics, and you can name it whatever you want.
The problem is that you won't be able to change that name, or that ability, ever again. It'll retheme itself, as far as is plausible, to your current Classpect thematics. I've heard a few people that could even invoke it in presession. Just another effect of Sburb's bad data-storage habits: this is something that should be stored as part of game data, not as part of one's Shiny.
Mechanically, the Custom Ability is treated as an inherent, small 1/chapter miraculous ability or inherent level 0 Affliction. It can thus be opposed by any Bond, Affliction, Auctoritas, or any ability that costs MP. (Or you can just bid 1MP to stop it.)
However, you can't add MP to the Custom Ability for any reason (you cannot give it more Strike or use it more often than 1/chapter).
Here are some examples of Affliction-based Custom Abilities:
[WD-40] (Level 0 Affliction: a native Dust player is pretty much walking lubricant; mechanical things just don't get stuck around them.)
[Dapper Duelling] (Level 0 Affliction: a native Time player always appears relaxed and put-together. This ability is very much like, and can be superseded by, Creature of Fable's Iconic.)
[Pointed Questions] (Level 0 Affliction: a native Law player's knives and swords do not dull.)
[Eternal Blue Sky] (Level 0 Affliction: a native Stars player does not panic when faced with the possibility of their own death.)
[Pushing Envelope] (Level 0 Affliction: a native Fate player always knows where the Parcel Mistress is.)
[Artemis] (Level 0 Affliction: a native Thief can always reclaim the arrows they've already shot; the points don't break and the feathers stay on.)
[Vegetable Drawer] (Level 0 Affliction: a native Doom player will always know how close food is to spoilage.)
And here are some examples of 1/chapter abilities:
[Representation] (1/chapter: a native Sight player can manifest a flipchart/diagram of something they know and want to explain to other people out of... well, basically nowhere.)
[The Spinner] (1/chapter: a native Life player can repair and preserve clothes made of natural fibers with a touch.)
[Line Feed Carriage Return] (1/chapter: a native Heart player can decide that they want the conversation to start over again, and thus resets everyone's mental state to "how they were feeling before we started talking".)
[Windows to the Soul] (1/chapter: a native Seer can look into someone's eyes and know what their most recent emote was.)
[Spark] (1/chapter: a native Flow player can start a fire just by snapping their fingers and pointing.)
[The Rest of Me Is Mere Appendix] (1/chapter: a native Mind player can turn off their perception of pain for just long enough to objectively evaluate the predicament they happen to be in.)