Finding a true 'ink demon' acting as a cursed protector is a pretty specific niche, but a couple of examples immediately jump to mind. The most direct one has to be 'The Ink Black Heart' series by Holly Black, where the protagonist literally makes a pact with a creature born from magical tattoos and spilled ink. It's bound to her bloodline, monstrous and terrifying to outsiders, but its primary function is to guard an ancient secret. It's less a gentle guardian and more a violent, possessive force that just happens to be on the protagonist's side, which creates a ton of internal conflict.
Another angle comes from webcomics and serial fiction, where I've seen the trope subverted. There's a story on Royal Road called 'Drowning in Ink' where the main character is an archivist who accidentally summons a scribble-demon from the margins of forbidden texts. This entity is made of shifting, wet ink and is initially a threat, but as the story progresses, it becomes clear its 'cursing' of the protagonist is actually a form of protection against a far greater corrupting force. The guardian aspect is earned, not inherent, and the 'curse' is the price for that protection—it slowly steals the protagonist's memories.
You could also make a case for certain interpretations of the Entity from the 'Bendy and the Ink Machine' universe in fanworks, though that's more unofficial. In a lot of fanfiction, the Ink Demon is reinterpreted as a tragic, bound guardian of the studio's lost souls, cursed by its own form to be perceived only as a monster. The protector role is framed as a reluctant duty it can't escape, which fits the 'cursed' descriptor perfectly.