In a nutshell, here are some main characters in Heidi with a quick character sketch for each:
Heidi, the protagonist, is a young girl whose kindness and open spirit bring joy to everyone around her. This starts when she moves in with her grandfather, a cantankerous old man, and through her joyful personality she brings light into his solitary life. Heidi's goodness does not change throughout the book, even as her situation evolves and she moves to the city. She is always eager to help others and goes out of her way to show friendship to all.
Alm-Uncle is Heidi's grandfather. At first he seems grumpy and aloof, only concerned with maintaining his solitary existence in the mountains with his goats. As the story goes on, however, we understand how deeply he cares for Heidi and how good his heart truly is.
Peter is Heidi's first friend, if you don't count the goats. He is a boisterous and energetic boy. Although he struggles in school, especially reading, and has some strong opinions, Peter knows a lot about caring for goats and living in the mountains. Heidi learns from him and gently guides him toward success in school as well.
"Grannie" is what they call Peter's grandmother. She is blind, but Heidi and Peter bring joy to her life when they spend time and read to her. Grannie values this time and does all she can to encourage Heidi and Peter.
Fraulein Rottenmeier is the housekeeper in the Frankfurt home where Heidi goes to work as a companion. She is quite strict and has no sense of fun. Heidi's exuberance only annoys her, and she spends her time being disagreeable toward Heidi.
Clara is the frail city girl whom Heidi befriends during her work as a companion. Through this friendship, Clara grows into a more adventurous and joyful girl, and she eventually finds healing with Heidi in the mountains.