It's long been speculated that chucking in certain combinations of runes together with your girlfriend/fiancé in the cube will have a chance of adding some hidden modifiers.

Especially among older players a common build seems to be adding DolEldHelIstTirVex to the cube when upgrading your fiancé, and some Ama/Sorc/Assa builds like adding EthLum or BerTirUmMalLum. Most guides however will generally recommend against using JahGulEth or VexHelElEldZodEth when upgrading a partner item.

Do note though that certain rune combinations may not be possible depending on the inventory size of your girlfriend base.