How can I make an unlikable protagonist likable?

[Karlin]

Read "Reaper Man", Terry Pratchett. He makes Death lovable.

 

[chongjasmine]

Try to create some strengths in him or her, be it something small, like he or she likes to help others.

 

[Graham J. Darling]

"Picaresque novel" might be what you're looking for. The Wikipedia article of that title lists many examples to learn from. To those, I add George MacDonald Fraser's "Flashman" novels 1969-2005, in which the protagonist's only redeeming features are "humour and shameless honesty as a memorialist"; but how he's so cleverly and outrageously inserted into actual historical events, like a scoundrel Forrest Gump, and how such a coward always comically gets away looking like a hero, also maintains reader interest.