Nathaniel Pope was on the stand for a second day of the murder trial and when asked about the day of Kemarni's death on June 5, 2018, said he had not caused any injury.
Kemarni's rib cage had been repeatedly fractured in the weeks before his death and he died of massive internal injuries just hours after visiting a doctor.
Pope, 31, of Evans Street, Wolverhampton, and Kemarni's 30-year-old mother Alicia Watson, of Raglan Road, Handsworth, both deny murder and child cruelty charges.
Defence barrister Jonas Hankin QC asked Pope: "Did you deliberately cause Kemarni any injury of any kind?"
Pope responded: "No I didn't."
After telling the court he had not seen or heard the assault that caused the boy's death, Pope was asked if he was "present in the room when Kemarni was fatally assaulted".
Sitting down in the witness box, he said: "No I wasn't."
Mr Hankin then asked: "Was it your intention that Alicia Watson should assault Kemarni with intent to cause him really serious harm?"
The defendant, who said he did not believe Watson posed a serious risk of physical harm to Kemarni, answered: "No it wasn't."
Refuting part of the prosecution's case in which a relative of Miss Watson said the boy had said "Tan Tan did it" when asked about bruising, Pope said everyone knew him as Tan Tan but Kemarni could not speak properly yet and always called him "Thanel" because he could not pronounce Nathaniel.
Pope also said he did use a chord to lock the boy in a bedroom but for only five minutes at a time as harmless fun to confuse the toddler about how the door was locked.
Pope said he was in the front room with Watson soon after the start of a 999 call in which she said "he's breathing quiet" and "he's not responding, his eyes are looking half open and they are just rolling around".
Mr Hankin asked Pope: "Was that an accurate portrayal of Kemarni at the time?"
The former chef at a Caribbean restaurant in Smethwick answered: "Yes it was."
The court heard Pope, who has admitted to being a heavy cannabis smoker, was arrested at 10.30pm on June 5 on suspicion of child neglect and was also detained in the early hours of June 15 on suspicion of murder.
Pope said he was in shock before an interview at which a prepared statement was given to police on his behalf saying he was "highly emotionally distressed" and did not wish to answer questions.
The trial continues.