Estel can be very hard to approach, but patience is a virtue in her case as she will allow herself to be tied after spending enough time with her, but she can also be taken by force fairly easily too. The best way to tell when she's ready to accept being tied is when she holds her right hand to her cheek as she isn't a very good liar. When it does come time to tie her, be sure to take advantage of her zettai ryouiki by tying her in a skirt-hiking shibari as her inner thighs are her weak spot. For her gag, give her something that emphasizes her hair and tie it over or under the hair. She does have certain rules though as she will never allow herself to be tied in her main chapel.