not quite. Yandere is just a character which is first lovey dovey, but because of their love, they become erratic, violent and/or psychotic (see Kaede from shuffle and kotonoha from school days)
That's why Rena (higurashi) is not a damn yandere. (rather yangire for some extent)
deretsun is just the opposite of tsundere (good guess captain obvious!): first lovey dovey, but turn into an unsincere and very obnoxious person.