Carol Black has quit the party exactly nine years after the UUP hailed her election to Banbridge Council as a "St Valentine's Day massacre" for the DUP.
She branded UUP leader Mike Nesbitt as "arrogant" and told the Belfast Telegraph: "I don't know if he is even a unionist."
For his part, Mr Nesbitt told reporters he is "relaxed" about his party's reaction to his plans, and remains committed to unionists and nationalists working together.
An unholy row in a rural church in County Fermanagh is the lead story in the Irish News.
The paper says a number of people "walked out" of Sunday Mass in protest after a priest accused named individuals of bullying him from the altar.
It quotes Fr John Halton apologising "unreservedly" for his outburst, and asking for forgiveness from his parishioners.
The Irish News also carries a fresh appeal from the family of a man murdered by loyalist paramilitary gang in a Lisburn pub 30 years ago.
Bin man Paul Bradley suffered a fractured skull when he was beaten with pool cues and a broken chair in the Tavern Bar in December 1986.
His brother and sister tell the paper they hope witnesses who were too frightened to come forward at the time might feel "safer" about giving evidence now.
Troubles legacy cases continue to feature prominently in all the local papers, with the Daily Mirror and others reporting High Court claims that former Prime Minister Edward Heath was "involved in the decision-making process surrounding the torture" of 14 men in Northern Ireland in 1971.
The 14 internees, who are known as the Hooded Men, want a a full inquiry into state involvement in the interrogation techniques used against them.
The Belfast Telegraph gets Alex Best's verdict on a new documentary on her late ex-husband.
The 45-year-old also dismisses claims that she ever suggested "supernatural" goings on in her 200-year-old cottage could be the ghost of George Best.
The Mirror features a very unusual Valentine's Day present that came as a complete surprise to one Belfast wife.
Murals of masked men are nothing new in Northern Ireland, but this one is in the shape of Holywood star Jamie Dornan, in his Fifty Shades Darker pose.
The paper says it has "appeared overnight" on a shop front in Belfast's North Street.
The resignation of an Ulster Unionist Party councillor, in protest at her leader's plan to transfer his second preference vote to the SDLP, makes the front page of both the Belfast Telegraph and the News Letter.