Learn French: Ce personnage a une cicatrice ; c'est quelqu'un de majestueux. - This character has a scar; he is someone majestic.

In French, when you want to describe an indefinite pronoun like quelqu'un (someone), quelque chose (something), personne (no one), or rien (nothing) with an adjective, you must place the preposition de right before the adjective. You cannot just say quelqu'un majestueux.

If the character were female, would majestueux change to the feminine form majestueuse?

No, it would not. The adjective following quelqu'un de (as well as quelque chose de, personne de, or rien de) is invariable. It must always remain in the masculine singular form, regardless of the gender of the actual person being discussed.

Why does the sentence use c'est quelqu'un instead of il est quelqu'un for 'he is someone'?

You must use c'est when the verb être is followed by a pronoun like quelqu'un, or by a noun modified by a determiner. You generally use il est or elle est only when followed directly by an unmodified adjective or a profession without an article (for example, il est majestueux).

Could we use personne instead of personnage here?