The defense that it was self-defense because the other party also used their hands is also not valid. Self-defense is an act that is unavoidable in order to protect one's own or others' rights against an imminent and unlawful infringement (Article 36 of the Penal Code). If the other party used violence first, it may seem that self-defense would be established, but "at the time the other party was injured, it is considered that the party who inflicted the injury was in an advantageous position, and it was not a desperate situation, in other words, it is highly likely that it was determined that there was no imminent threat" (ibid.).
So what actually determines who is the perpetrator and who is the victim? The key is calling 110.
"The police rush to the scene with the preconceived notion remove background image that the person who reported the incident is the victim. Therefore, unless it is clear that one party is lying on the ground bleeding, they will simply assume that the person who reported the incident is the victim and the person who did not report the incident is the perpetrator, and proceed with the case. In cases where both parties are equally likely, the person who reported the incident first tends to have an overwhelming advantage." (ibid.)
On the other hand, it can be troublesome if the other party reports it first. Even if you tell the police officer at the scene that you are the victim, they may not listen to you. It is possible to file a report, but there is a possibility that they will be reluctant to accept it, or that you may be perceived as abusing the system for the purpose of revenge against the other party, which may work against you. Once you are labeled as the perpetrator, it is difficult to change that later. The only way to protect yourself is to report it first.
"If the other party reports you first, one option is to contact a lawyer right away. If all goes well, you may be able to overturn the victim/perpetrator structure by filing a complaint yourself." (Same source)