Samkhya's twenty-five tattvas divide sharply into two kinds of thing, and purusha is the whole of one kind by itself. The other twenty-four form a single chain: prakriti unfolds into intellect, intellect into ego, ego into mind and the ten organs and the five subtle elements, and the subtle elements harden into the five gross elements that make up the physical world. Every link in that chain is prakriti in a different guise, and every link can in principle be traced back to the equilibrium of the three gunas disturbed at the start of a cosmic cycle. Purusha is not a link. It does not evolve from prakriti, nothing evolves from it, and it is not made of the gunas at all.
What purusha contributes to the twenty-five is presence rather than substance. Samkhya's classical image is a dancer performing before a seated audience: prakriti is the dancer, purusha is pure witnessing, and the appearance that purusha is involved in the performance comes only from proximity, the way iron near a magnet seems to move on its own. Multiple purushas are posited, one for each apparently separate stream of experience, which is itself one of the system's more debated commitments, since an inactive plural consciousness raises the question of what distinguishes one purusha from another if none of them does or has anything.
Counting purusha as the first of the twenty-five tattvas, alongside twenty-four items that are all stages of one unfolding substance, is the clearest sign of what the enumeration is actually for. It is not a list of ingredients in the world; it is a map of everything that exists, sorted into the one thing that watches and the twenty-four things, all continuous with each other, that are watched.