What makes a character a "good" character?

depends

I would say Motivation and Response to Motivation. A character has agendas and obstacles to achieving them. Their personality is revealed in that pursuit: how they go about the task,how much effort they put into the task,and how they deal with the frustration of not succeeding at it. All these actions are coloured by their skills,intelligence (or lack thereof),and what passes for their past. And the enviorment they find themselves in will dictate their choices,and how far from their comfort zone navigating through it will be.

For example, Fagan from Oliver Twist,Tony Soprano from the Sopranos. Two Crime Bosses,wanting to get rich. Different methods,underlings and tools,different obstacles. But both with opportunistic criminal minds. Now plop them both on a desert island: how would they respond to the new paradigm? Such is character,you have a vague idea based on what you know of them.

a mix of all of this.
i'd say the first is being relatable. then comes development,after that actions and behaviour and lastly background.

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>>9187583
A nice rack.

>>9187708
also agreeing with this. you should know what your characters underlying value is to have coherent reactions. what are they striving for and why. they themselfes do not need to know it nor does the reader. but the author should be aware of it.

for example, some dude being on a crusade trough some monster ridden forrest might make an entertaining story. it gets better if you add a goal. for example, find a deserteur.
it starts to get relatable when you also add why he has that goal. maybe revenge. now you'd need a reason. for example, that guy killed the prots little brother for selfish reasons. now the resder starts to feel with the prot. after that, you can also add another layer. base values. maybe your prots most valued things are family and honor. which would be a big driving force to make him hold up even if things get really uncomfortable during his journey.

>>9187583
being intelligent, nihilistic, and having a wicked sense of humor

>>9188738
hue