Louisa is Thomas Cahill's first wife. She is only mentioned in The Black Book of Buried Secrets. She is from Portugal. She is the reason his sister Katherine Cahill left him and stole one of his Clues. Thomas and Katherine had been running from Luke Cahill, their brother, and they wound up in Portugal , where Thomas fell in love with and married her. After 3 years, Katherine became angry because they weren't doing anything so she ran away. Thomas left Louisa to chase Katherine, and she is never mentioned again. After Thomas stopped chasing Katherine, he married a Japanese woman named Keiko and built the first Tomas Stronghold in Tokyo.

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