The 35-year-old Welsh former model, who has a love heart tattoo on her rear, surprised the remaining six contestants on Saturday night as they found her waiting for them in the reward room.
Channel 4's website describes says new contestant Lisa's ideal job would be working as a soap opera actress, but most recently she has been working in a clothes store while completing an Open University course in social sciences.
According to a Channel 4 spokeswoman: "'Big Brother' is always looking for new twists during the series, and this will be another little challenge for the housemates."
Channel 4 are hoping the 35-year-old will boost ratings and, just as importantly, voting on the show, which has fallen to a new low.
Just 676,109 votes were cast by phone, interactive TV or by text message on Friday, with Tania picking up 72% of the total. Viewing figures, which had risen to 5.5m when African housemate Gae entered for a week, dropped to 5.2m.
This compares with the 3.5m votes cast in last year's 'Big Brother' when contestant Adele was voted out of the house.
"The viewing figures are good and so are the figures for the website, so we're happy with it. 'Big Brother' is a different experience every year," the Channel 4 spokeswoman said.
The other two contestants, Nush and Steph, who were also up for eviction on Friday polled 99,104 and 90,954 respectively.
New 'Big Brother' contestant Lisa can not nominate or be nominated in the coming round of evictions, but if the four boys left in the house continue to nominate the women, Channel 4 will see either Nush and Steph voted out this coming week.
Although she has arrived on day 37 of the 64-day show, Lisa has an equal chance of winning the £70,000 prize.
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