Krobus

Friendly shadow person who runs a shop in the Sewers.

Krobus is a villager in Stardew Valley who lives in the Sewers beneath Pelican Town. He cannot be married, but later in the friendship the player can invite him to move into the farmhouse as a roommate.

He is a shadow person, the same kind of creature as the Shadow Brutes in the Mines, and the only friendly monster in the game. He keeps a shop in the Sewers that is open at all hours, including festival days, with a fixed stock and a rotating one. He dislikes dwarves, citing a war between their peoples that lasted a thousand years and only recently ended.

Quick Facts

Role

Villager
shopkeeper
possible roommate

Home

The Sewers

Birthday

Winter 1

Friend

Wizard

Facts from the source article.

Schedule

Krobus can always be found in the Sewers once you have the Rusty Key, and his shop there is open at all hours every day, festival days included. Besides a fixed stock that includes the Void Egg, Void Essence and Solar Essence, it sells one rotating item a day: Slime on Mondays, an Omni Geode on Tuesdays, a fish or a Magnet on Wednesdays, Mixed Seeds on Thursdays, an Iridium Sprinkler on Fridays, a cooked dish on Saturdays and Bat Wings on Sundays.

He cannot be married, but if you are single he will move into the farmhouse as your roommate three days after you give him a Void Ghost Pendant; he then leaves the house only on rainy days, to stand on the front porch, and a chest in the Sewers sells his usual items.

Source: Stardew Valley Wiki: Krobus (Schedule)

Gifts

Loves: Diamond, Iridium Bar, Monster Compendium, Monster Musk, Pumpkin, Void Egg, Void Mayonnaise, Wild Horseradish; also all universal loves.
Likes: Gold Bar, Quartz, Seafoam Pudding, Strange Bun; also most universal likes.
Hates: most universal hates.

You can give him up to two gifts a week, plus one on his birthday (Winter 1), when a gift counts eight times.

Source: Stardew Valley Wiki: Krobus (Gifts)

Heart events

Three hearts: He mails you the recipe for the Dark Sign, in a letter he wrote with the Wizard's help.
Fourteen hearts (the beach, 8 PM to 1 AM on a dry day): Watching the Moonlight Jellies from the docks, he meets a sea monster that lets him ride on its tentacles.

Source: Stardew Valley Wiki: Krobus (Heart events)

What players say: Player verdict on Stardew Valley — Rotten Tweets