author
Best known as the pen name of Finnish writer Elli Hällström, this early 20th-century author wrote for younger readers and brought everyday school life into fiction. Her work has also been preserved through Project Gutenberg, helping new readers discover it today.

by Armi Vainio
Armi Vainio was the pen name of Elli Hällström (née Åkerman), a Finnish dentist and writer for young people. Sources found during this search identify her as a Finnish children's and youth author and give her life dates as 1873–1955.
The name Armi Vainio appears especially in connection with youthful fiction, including Kaarinan kouluvuodet, a work available through Project Gutenberg. That surviving availability suggests her writing continued to matter well beyond its original publication era.
I couldn't confirm many personal details beyond her identity, profession, literary focus, and dates from the sources retrieved here, so this overview keeps to those verified basics.