if you do it right you can make people LOVE a character with only one pannel.
Sometimes a sketch can get people interested if the design stand out.
Of course, the personality is a key element too. Sometimes just getting the character to talk (if he talks) or make a gesture can work wonders.
Sometimes the way people receive some characters can be a surprise.
Fun fact: one of the most popular villains in my comic has appeared in only 2 pannels for now.
So, there is no minimum for sentiment to set in.
The key is to make your characters feel "real", and for that a solid and consistent characterization is key. Even if they change in some ways like having some character development, is important the transition makes sense for them.
They don`t need to be realistic, but they must be beleivable inside your story.