Stacey Flounders, who is the mother of Johnson's year-old daughter, has begun giving evidence as part of the 28-year-old former Sunderland winger's defence at Bradford Crown Court.
Johnson has admitted kissing a 15-year-old girl in his car after grooming the teenager using social media.
But the player denies that more serious sex acts took place between him and the girl in his Range Rover on January 30 last year.
Miss Flounders, 26, was asked by Orlando Pownall QC, defending Johnson, about the future of their relationship.
She replied: "We're just going to remain friends."
Earlier, Mr Pownall asked her why she decided to remain with the player after his arrest.
She replied: "Because he was honest with me and I knew he was telling the truth. We'd just had a daughter. I wanted us to be a family."
Asked about messages exchanged with Johnson on the day of the incident with the girl, Miss Flounders admitted she suspected him of being unfaithful at the time.
Mr Pownall asked: "Did you suspect that he was cheating on you?"
She replied: "I did, yes."
When the barrister asked if she had any "hard and fast evidence", she said: "No".
And asked about when Johnson was arrested at their home at 8am, she said: "My head was all over the place. I didn't know what was going on."
She said that after his arrest, her partner said the girl's name and added: "I gave her a couple of shirts."
Miss Flounders said: "I asked how old she was. He said she was 16."
She had already told the jury that she had been in Twitter contact with the girl, whom she recognised as one of the people who waited for Johnson after home games.
Miss Flounders said she even wished the teenager a happy birthday on Twitter in 2014.