Kartikeya is born to solve a problem. The demon Taraka has a boon that only a son of Shiva can kill him, and Shiva at that point is a widowed ascetic with no intention of fathering anyone — which is why the gods go to such lengths to bring about the marriage with Parvati, and why the conception, when it comes, is handled by intermediaries. Agni carries the seed and cannot hold it; it passes to Ganga, who releases it into her current, and the child is finally deposited in a thicket of reeds on the Himalaya.
There he is nursed by the Krittikas — the six stars of the Pleiades — and because he takes milk from all six at once he grows six faces to do it, and takes the name they give him.
ततस्तु देवताः सर्वाः कार्तिकेय इति ब्रुवन् । पुत्रस्त्रैलोक्यविख्यातो भविष्यति न संशयः ॥१-३७-२५॥
“Then all the gods said: ‘Let him be called Kārttikeya. Beyond doubt this son will be renowned throughout the three worlds.’” — Valmiki, Ramayana 1.37.25, tr. Mythos Atlas
Within days of that he is anointed commander of the celestial armies and kills Taraka, which is the whole of the commission. His other name, Skanda, attaches to the manner of his conception; in the south he is Murugan, and there he is not a subordinate war-god at all but a major deity in his own right, with the six great temple sites of Tamil Nadu and a following that treats Shiva’s son as the centre rather than the appendage of the family.
He rides a peacock and carries the vel, a spear given him by his mother, and his banner bears a rooster. Iconography usually gives him six heads and twelve arms, though a single-headed youthful form is at least as common in southern bronze. Where his brother Ganesha is the god one approaches before starting anything, Kartikeya is the one invoked when the thing has already started and has to be won.