The tactician in a story is a character who makes plans. Usually they are the only ones who decide where their group of characters will go and how they will go about doing that meaning that if another character were to make such a decision it would come at a great surprise. This doesn’t have to be the case. There does not need to be one designated tactition per group of chara . This role can be filled by many characters at the same time. Of course this is the case for most character roles.
Any character can be a tactition. The image that comes to my mind when I think of such a character I think of a highly educated individual or someone who is just extremely gifted. A tactician does not need to be a genius or someone who has gone to school per years. All the character needs to be a tactician is good at making plans.