Who was first person in history to do complete teshuva (repentance)?
ויצא קין יצא כמעלים מבוראו ונגלה לו הקב״ה וא״ל גדול עונך מנשא כיון ששמע קין התחיל לעשות תשובה ואמר גדול עוני מנשוא ואמר אתה טוען עליונים ותחתונים ונקראת נושא עון אמר לו הקב״ה אתה עשית תשובה אבל מחצה עד שהוא יוצא פגע בו אדם א״ל ומה יצא בדינך א״ל אלמלא שהודיתי הייתי אבוד מן העולם באותה שעה תמה אדם ואמר טוב להודות לה׳
Or, in my own, loose translation:
"Kayin went out" [a quotation from 4:16] — he went out like someone hiding from his creator, and God appeared to him, saying "Your sin is too great to excuse". When Kayin heard this, he started doing t'shuva, saying "My sin is too great to excuse" and "You bear what is above and what is below and have been called the one who excuses sin". God told him "You did t'shuva, but halfway".
As Kayin was leaving, Adam met him and said "How did your court case come out?". Kayin told him "Had I not admitted, I'd be gone from the world". At that time, Adam marveled and said "It is good to admit to God".
Avraham was Chozer Beteshuvah from a life of Idolatry to recognizing the true Boreh Olam (creator of the universe). See Rambam Hilchos Avodah Zorah.
(a) (R. Yirmeyah ben Elazar): The years that Adam ha'Rishon was in Niduy (excommunication due to his sin) he propagated damaging spirits. "Va'Ychi Adam Sheloshim u'Me'as Shanah va'Yoled bi'Dmuso k'Tzalmo", implies that before this, he fathered things not in his image.
(b) Question (Beraisa - R. Meir): [After he sinned] Adam ha'Rishon was a great Chasid. When he saw that death was decreed on the world through himself, for 130 years he fasted, separated from his wife (Kayin and Hevel were already born - Rashi Bereishis 4:1) and wore belts of fig shoots on his flesh [to afflict himself].