You should say the greatest worthy rival to Celtic, and there is club team called Rangers. I am the same as Celtics, and Glasgow of Scotland is a stronghold. Celtic is club team receiving Scotish (Gael) majority support of the progenies of the Celts in Catholic system. In Ireland, Northern Ireland, it is popular probably because I assume it a club with the same culture background.
The F.C. called Rangers is classified in Anglo-= Saxon system by the objection, and there are many federationists (a unionist) who are a sympathizer of the U.K. alliance of the state religion associate English for a supporter.
There is a considerably complicated background so that the religious relations of the politics of the race are included as above about Celtic and Rangers. Therefore there is the problem that it is difficult to tell it simply only at the side called sports. The relations of such a rival are called "the old farm Derby".
We have the reputation called the most dangerous Derby in Celtic and Rangers and the Glasgow Derby all over the world. I become the greatest event to be excited with both athletic supporters very much from old days. I heated up so as to have sent a dead person in the past, but in late years calmed down to some extent. But medical care is necessary even if I put it after a match at present.
It is different from Japan that there is a rival of two teams in one town called Glasgow. I am wearing a uniform after a match was over and walk the city because one's favorite team won and must avoid that you enter a shop so that there is danger to rub the feelings of the partner team up the wrong way.
In the world of sports, there is a rival in any kind of team, but after all Rangers can be said to be the best worthy rival for Celtic.