Most Helpful Guy
If you truly regret kissing him, then I would maybe consider leaving that part out. That can actually crush a man mentally. Just tell him that you are not interested in a relationship with him, and that you wish him to find someone great.
Most Helpful Girl
You can tell him nicely but inevitably as an ending result, this "nice guy" who is worth it will now have his guard up and that will only make it worst for the next girl that comes around that might actually like him.