A relative clause referring to "whoever..." isn't necessarily a relative clause of characteristic. In fact, it usually isn't. And a relative clause of characteristic can refer to a specific person.
Relative clauses of characteristic are often a cause of confusion because the term is so vague. And they can be difficult to explain clearly.
I'll offer an attempt at clarification from a book I wrote:
The general definition of a relative clause of characteristic is a relative clause that describes a person or thing as being
of such a character as to do or undergo this or that; as being the
kind of person or thing that does or would, generally or in a specified situation, do or undergo this or that. In other words, a relative clause of characteristic does not so much state a fact about what a person or thing does, but it tells us that they are prone, able, willing, or expected to do so, or that it is part of their character. This definition, however, is arguably vague and open to (mis)interpretation. To give you a better idea of what it means in practice, here is a list of a few situations where relative clauses of characteristic typically occur:
- 1. Situations where a point is being made about the very existence or non-existence of the kind of person or thing described by the relative clause. Such sentences typically involve phrases like nēmō/nūllus est quī…, ūnus est quī…, sunt quī…, ūnus inventus est quī…, nēmō aderat quī…, ūnum repperimus quī… Note that all of these expressions except those where existence or availability is negated (with an antecedent like nēmō, nūllus, nihil…) sometimes take the indicative, with a different (more factual) nuance; but the subjunctive is very frequent.
- 2. Relatedly, situations where the existence or availability of a person or thing of such and such a kind is not stated as certain but as potential, contingent, etc.; for instance, in sentence 4 [Volō quaerātis magistrum quī linguam et Latīnam et Graecam absolūtē sciat] the speaker wants the addressees to look for a teacher, and wants the teacher to be of the kind that knows both Latin and Greek perfectly. Whether such a teacher will be available for the speaker depends on the addressees’ finding such a teacher in the first place. If the indicative had been used in the relative clause, it would have meant that the speaker already knew a certain teacher with those qualities, and wanted the addressees to look for him specifically: "I want you to look for a (certain) teacher who (I know) knows both Latin and Greek perfectly, namely such and such person."
- 3. Relative clauses that are linked to an adjective (or several adjectives) by a word meaning "and" or "but", e.g. Pūblius vir est bonus et quī amīcōs numquam dēserat, "Publius is a good man and (a man of the kind) who never abandons his friends".
- 4. In statements affirming or denying that a person or thing is one that tends to do so and so as part of their or its character. This often (though not exclusively) happens in connection with a form of is, like in Sentence 3 [Nōn sum is quī amīcōs dēseram].
- Relative clauses of characteristic may be viewed as a type of consecutive relative clauses.
