Per Alaska:

Can I upgrade to First Class with my companion fare?

Yes. When traveling on a companion fare, you are eligible for all upgrades we offer. You can upgrade using miles, elite + companion upgrades, guest upgrades, or paid upgrades that may be offered at check-in. You and your companion must travel together on the same reservation, in the same cabin, and upgrade space must be available for both guests.

We can’t separate guests traveling on a companion fare into separate cabins, or upgrade you at separate times. Except for the companion upgrade allowed for our Gold and 75K elite members, both guests must use the same type of upgrade. (For example, a guest upgrade and an upgrade using miles can’t be combined.)