Ilmarë was among the most powerful of the
Maiar. She was the handmaiden of Varda and lived in the highness of the
sky. Eönwë, the herald of Manwë, was her brother. These
two were the eldest of
the children of the Valar. Eönwë was a mighty warrior but
Ilmarë wasn't even interested in anything that concerned war. She
was first named Erinti, then Ilmar, then Ilmarë.
I see Ilmarë as a serene and delicate
personality. The Eldar considered her as one of the loveliest of the
Maiar. She maybe wasn't interested in war, but she was certainly very
powerful. To conclude, I think Ilmarë was jovial, warm, fervent,
but wasn't the kind of girl to upset.
References: Silmarillion, Lost Tales
Tolkien considered calling Ilmare and Eonwe the children of Manwe and Varda. This did not make it into the Silmarillion, as he seems to have decided that such beings did not have children. It does not specifically say, however, that the Valar had no children. All it really says on this is:
"Chief among the Maiar of Valinor whose names
are remembered in the histories of the Elder Days are Ilmare, the
handmaid of Varda, and Eonwe..."
--"Valaquenta" Silmarillion JRRT
An interesting but unreferenced comment from David Day (therefore
non-canon unless we find a corraborating Tolkien reference) suggests
that Ilmare handles part
of
the lightning:
"Ilmare, who throws down her spears of light
from the night sky, is chief of the Maiar maids. She is also handmaid
to Varda, the Star Queen."
--"Maiar" Tolkien, the Illustrated Encyclopedia by David Day