The Voidwalkers and Umbra had been on friendly terms with each other for years, and this resulted in frequent partnerships. This was all thanks to the efforts of the Assassin Walker.

They mostly partnered in technical support and intel exchange; Umbra had never asked the Voidwalkers to lend them a hand in assassinations. 

When Lady Assassin Walker's identity as one of the Voidwalkers was discovered, Umbra sent her, their legend, invites to return to the organization. They even went as far as offering her the position second direct-in-command, but she rejected them, stating that she wanted to be a housewife of the man she loved. According to the codes of Umbra, this constituted as a perfectly legitimate reason for an assassin of her caliber to retire, so in the end, the only position Lady Assassin Walker accepted was being Umbra's spokeswoman.

The girl definitely did more than just promoting the Umbra brand, though. In her spare time when she was not busy pining for Baiyi's attention