Technically, Ysabel's mother's maiden name isn't Ivena; it's s'Ivena. However, this name is the only important "I" word I can think of for the Season Avatar series. This name also allows me to talk about how Challen surnames indicate family status:
Farmers and laborers have plain surnames of a single word, such as Jenna Dorshay and Kay Seltich.
Middle/upper middle class people have a prefix (a single letter separated from the rest of the name by an apostrophe and pronounced separately) in front of their name, such as Mattie s'Ivena.
Nobles have a short word in front of their surnames, such as Gwendolyn lo Havil.
Finally, royalty from the Fip Dynasty combine the surname of Fip with the country where they reside, such as Challen. To indicate they're royalty, they use "ro" as an abbreviation. Therefore, the king's brother is Lex ro Fip-Challen.
Avatars are born into all classes of society, but they're ennobled when they're officially recognized as Avatars. Jenna, Ysabel, and Kay change their names in different ways. Jenna was briefly married, so she combines "dor" from her maiden name of Dorshay with her married name, "t'Reve," to become Jenna dor Treve. Ysabel has a Challen mother and Selathen father. In marriages to non-Challens, the Challen surname is passed down as a middle name, so Ysabel's full original name is Ysabel s'Ivena Lathatilltin. She drops her father's surname (they don't get along very well) and uses "Ava" in front of her mother's name, becoming Ysabel ava Sivena. Kay also adds "Ava" to indicate her status as Kay ava Seltich.
Hopefully this naming system isn't too confusing. At least I won't give you a quiz at the end of the month.