Appearance and AbilitiesEdit
She had big blue eyes and was dreamy, with few friends even in her own Ajah. She was 5'5 and slight of build. Mesaana also bore a passing resemblance to her, enough that only a little Illusion was needed for Mesaana to impersonate her. These points made her a great candidate for Mesaana to impersonate.
Danelle probably was a High Ranking Sister (and so she was also quite strong in saidar).
Clues in the main series that pointed to Danelle's strength: firstly it can be assumed that Mesaana opted to take the identity of a featureless sister that would still be fairly influential, not one doomed to obey to the majority of the other Aes Sedai.
Secondly it can be considered the fact that Danelle occupied for a while some relevant positions: she was in the group of leading sisters that supported Elaida's coup, and she was also chosen to be in the personal inner council that Elaida kept for a while.
To take the identity of Danelle obviously Mesaana needed to mask a good part of her strength in saidar.
This was later confirmed by the WOT Companion, which gave her strength as 17(5). This is as strong as many Sitters (e.g. Janya Frende).
Born in 899 NE, she went to the White Tower in 914 NE. Spending nine years as a novice and seven as Accepted, she was raised in 930 NE. She would have progressed faster had she not always been going off into a dream. She spent much time in introspection, and could drift off while talking to someone.
Danelle spent a lot of time out of the Tower - in fact, as soon as she gained the shawl she left - and had no close friends as she was always lost in thought or books. She was seldom in the Tower for more than a few months at a time, and at widely-spaced intervals. This meant she was quite unknown, and Mesaana took advantage of that to impersonate her. She also never had a Warder, which helped.
Some time before the coup that dethroned the Amyrlin Siuan Sanche, Danelle Sedai was in charge of some masonry works in the White Tower Library. Checking the counts for that work, Siuan noted that Danelle hired too many workers for that job, and she wanted to scold the Sister about that. But the Amyrlin never had the opportunity because the men were not masons, but mercenaries that in a short time participated at the coup d'etat.
Danelle was one of the sisters with Elaida that came to arrest Siuan after the Hall had removed her as Amyrlin. Danelle was initially on Elaida's inner council  and for a while she continued to have some importance writing reports to Elaida about the attitudes of various rulers towards her . She did not remain in this position long, however, and was soon pushed aside, though Alviarin doubts she even realized it.
Later was discovered that Mesaana at some stage took the identity of Danelle. Danelle had been squeezed of all the information necessary for Mesaana to impersonate her, and been killed; and so therefore Danelle never actually appeared in the books. The Danelle that helped Elaida was already Mesaana in disguise. Danelle had few friends inside the White Tower, which helped explain the ease with which Mesaana took her place and identity. Because Danelle had been so friendless and solitary, and because Mesaana had resembled her so much, little Illusion was needed for the impersonation.