Comma or semicolon?

He seemed like a cat, one that was afraid of strangers.

Comma

http://writingcenter.unc.edu/handouts/semi-colons-colons-and-dashes/

In this sentence, you would use a comma.

To determine whether to use a comma or semicolon in a sentence like this, you need to consider the relationship between the two clauses. In this case, the phrase "one that was afraid of strangers" is providing additional information about "He seemed like a cat." It is acting as a dependent clause, also known as a subordinate clause.

When a dependent clause follows an independent clause (the main part of the sentence), you generally use a comma to separate them. The comma serves to signal the relationship between the two clauses and helps with the clarity and flow of the sentence.

Therefore, in this sentence, it would be grammatically correct to use a comma after "cat" to separate the two clauses: "He seemed like a cat, one that was afraid of strangers."