As others note, it is classified in the tree of life. We know approximately what kind of bacteria it evolved from, and mitochondria themselves constitute their

As others note, it is classified in the tree of life. We know approximately what kind of bacteria it evolved from, and mitochondria themselves constitute their own divergent branch of the tree of life (there are many longitudinal studies of mitochondria across species). But if you wanted classify them based on functionality rather than evolutionary history, I'd say they're more like viruses. They have only a handful of genes themselves, and exploits the nucleus' genetic material for all the other proteins it needs to function.