Like everyone else said, any likeable character first needs to come across as a person instead of just a plot device. What exactly makes me feel this way is a bit hard to pin down.
Some possible factors:
They should have opinions on each other, on the world, on the MC’s actions that don’t directly affect them. They should have strong internal drives and beliefs, and will act on them independently.
It is very annoying to have a character’s every action and thought to be MC-centric or MC-endorsed.
To give me the chance to like them, the MC should have the opportunity to have meaningful and revealing conversations (like the campfire talks in DAO) and interactions with them. They should ideally have heavy and irreplaceable interaction with the main plot, and/or have plotlines of their own.
It’s also wired into us that we tend to like the people who like us.
For me to have the possibility of caring about about them, I need sufficient immersion in the world, and MC.
They also need some basic capability for empathy and respect for the value of life. Not asking for a saint or anything, but I can’t like someone whose every action is for their own benefit, or who don’t consider the casualties of their actions.
It helps when they are shown (not just described) as competent, and have earned that competence.
It also helps when they undergo character growth, in personality/capability/situation.
Most of my favorite characters are RPG companions. IF-wise, my favorites are Finch and Alexandra from A Study in Steampunk, Semyru from WWU, the Rangers from FH.
A really memorable counter-example is the ROs from the Hero Unmasked. Their personality, morality and entire underlying character is dependent on something as whimsical as whether the MC likes them. It made it extremely difficult to see them as people.