Appearance and AbilitiesEdit
She had big blue eyes and was dreamy, with few friends even in her own Ajah. Mesaana also bore a passing resemblance to 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). This was confirmed by the WOT Companion, which gave her strength as 17(5). This is as strong as many Sitters (e.g. Janya Frende).
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.
Danelle spent a lot of time out of the Tower and had few friends. This meant she was quite unknown, and Mesaana took advantage of that to impersonate her.
Some time before the coup that dethroned the Amyrlin Siuan Sanche, Danelle Sedai was in charge of some masonry works in the 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 that along Elaida 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. It can be assumed that Danelle did not survive this and so therefore never actually appeared in the books. Danellehad few friends inside the White Tower, which helped explain the ease with which Mesaana took her place and identity.