When she snuck out of Goa in Luffy's tiny boat, she avoided to help out fights, such as with Alvida, Morgan, Kuro and Don Krieg or put simply in the Romance Dawn Arc. The reason for this was due to her not knowing one bit how to defend herself due to her handicap plus fearing that any wrong action could jeopardize the crew in battle. Even with DF powers Ydel didn't participate in any battles until Arlong Park. This would not only be the first time Ydel reveals she has DF powers but it's also her first time to participate in battle and it is when she helps Usopp defeat the Chew (of Arlong's crew). Since the Arlong Arc, Ydel confidence had a positive boost and tries to be more into fighting but even with all that, she still doesn't step into battle unless she doesn't have a choice or if it's to survive, but the main reason is because lack of control of her DF and without any training she of her DF she could easy and unintentionally.
However, by the time the Strawhats entered the GrandLine, and have traveled through many islands and comparing the battles against the ones they've had on East Blue, Ydel attempts to take a more active role, wanting to help out the crew, rather than be known as a liability for not only her age but her blindness: she scavenges items the crew might need and find useful, helps Luffy and Sanji on Drum Island on getting Nami to a doctor and participating in battles more often.
This seemed to continue until Enies Lobby Arc, where she found herself to be more of a burden than help in a situation of life and death of a crew member, mentioning if she would have been stronger, then she wouldn't have to sit back and watch a friend get hurt or worse, and due to this, Ydel develops a desire to fend not only for herself to prove her independence and usefulness despite being blind, such as asking Zoro and Brook to teach her how to wield a weapon.
By Sabaody Arc, her fighting skills using her DF show no use against the Pacifista and she along with the rest of the crew are poofed away by Kuma. And during Clover Island, upon arriving and wandering out of the forest where she landed to the city she used what she knew in order to survive, which meant running and hiding from Saint Julianne and her bodyguards. Having come close with near captivity and death, her longing for self-defense increases as she mentions to Umy and during her time on Clover Island and to do whatever it takes to survive.
However by meeting Dai and him teaching her how to use her new sight and to better control her DF powers, Ydel changes significantly both emotionally and in battle. Putting her fears and self-doubt aside, she becomes battle-hardened, from two years under Dai's teachings. Her control over her DF power have improved and she has adapted to her new way getting around easily (Ydel had Observation Haki before but it wasn't well developed) and even taking Dai's ways of not killing as every life is precious.
In Fishman Island Arc, Punk Hazard Arc and Dressrosa Arc, Ydel could fend off by herself in the most cases and now even participating on full on battles, at times with a little help from the Strawhats or even helping them in a sticky situation such as facing Sugar risking getting turned into a toy and getting in the midst of mayhem to help Luffy up during his battle against Doflamingo. Even though now being capable of defending herself, Ydel still helps in small things such as finding useful stuff for the crew and offers Momo to use his DF powers the same was Dai taught her.
Well that's all i have for now and may add more in the future.