Attributes can be arrays by removing the type and adding array: true:
class User
include ActiveModel::Model
include ActiveModel::Attributes
attribute :name, :string, default: "n/a"
attribute :akas, array: true, default: []
end
User.new(name: "Luke", akas: ["LSW", "Skywalker"]).attributes
=> {"name"=>"Luke", "akas"=>["LSW", "Skywalker"]}