Oh, so it's the classic case of a mangaka not knowing how to depict a relationship, so they throw in another heroine to have an excuse to clog up the plot with all kinds of insecurities and misunderstandings, because wouldn't it just be a tragedy if the characters actually had no excuse to not just fall mutually in love and confess. Gotcha.
(My avatar is related... there's a "tomboy childhood friend" character exactly like this new girl in the manga it's from, Houkago no Goumon Shoujo. Serves no purpose other than to act possessive of the male MC.)