When using the kanji bearing the meaning of "friend", if one chooses to pronounce it "yuu" rather than "tomo", then it's "yuu" with a long "u" and not "yu" as the yu in Yumiko. So that would be Yûmiko. It may exist, I can't be sure, but I don't think it is really used.
Yumi, when used as a name, refers to "yu" meaning bow, and "mi" meaning beauty, to combine together making "beauty bow," or "beautiful bow. [noted -ed]
Again, many possibilities for this name (depending on the kanji)."yu": -kind -freedom (I think they also use this character for "bow", but I'm not sure) -together/friend (rare, but I did meet one Yumi who used this kanji... though the name would normally be pronounced Tomomi in this case) -bear (produce... as in produce fruit) -kind/smart"mi" -beautiful -fruitVariations with Yumi: Yumi Yumie Yumika Yumiko
I personally love the names Yumi and Yumiko. Though "Yumiko" is not on this site, and on another I found it meant "beautiful child." I thought it would have meant "bow child" or something, because of the use of the suffix "ko" which usually means "child."