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Here is the full sentence including the word.
小気味いい矢吹丈と、ボクシングだけを想う老残の丹下段平、そしてチャーミングな仇役力石徹。
I found that the first Kanji 「仇」is 表外漢字 and an alternative form of 「敵」. Then, I am curious why the writer did not use the later one which would be easier to read for Japanese reader.
仇 is not an alternative form of 敵 (there's a word 仇敵【きゅうてき】). And aside from it, not being 常用漢字 doesn't mean immediately the kanji is rarely used. Kanji frequency distribution usually shows a long tail, and the rank varies depending on the source collection. According to @scriptin's investigation, 仇 is at #1951 among novels in 青空文庫, as well as #2280 in Wikipedia JP, so you can see it's a pretty close loser to 常用漢字 (now has 2,136 kanji).
In my personal experience, 仇 for the word かたき ("foe; rival; who should be avenged on") is a fairly well understood, or perhaps more preferred way of writing, partly because 敵 stands for another well known reading: てき ("enemy; opponent").
