Goldfish

Three seconds of memory and a full heart every single time it sees you. Nothing bad ever sticks.

Adopting one takes a tap and no account. It grows through five stages over about sixty days, it cannot get sick and it cannot die, and it is never sad at you for being away.

The wallpaper is 1080 by 2340, which fits nearly every phone made since 2019. The card is an animated GIF, so it works anywhere a picture does.