Alterous is described as neither being (entirely/completely) platonic nor romantic, & is an attraction best described as wanting emotional closeness without necessarily being (at all or entirely) platonic &/or romantic, & is used in the place of -romantic or -platonic (so say bi-alterous instead of bi-romantic).
Someone can be both alterous & romantic &/or platonic & can have varying degrees on attraction, ultimately feel discomfort / unease / or just a sense of inaccuracy in calling it wholly romantic or platonic.