To gain a Rank up, experience (or EXP) is required for each character. Experience can only be acquired by absorbing the Id of folks. If the Ids of multiple folks are absorbed at the same time, an EXP multuplier will be shown under the MC bar. However, if a folk is defeated, no experience will be given.
There is only one advantage for Ellen and Keats to Rank Up, which is to increase their HP.