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.

Daley dived twice from 10m and once from 3m, while Gallantree completed the reverse routines.
They scored 434.65, with Ukraine (426.45) winning silver and China (425.40) bronze in Kazan, Russia.
Britain won bronze on Tuesday in the synchronised 3m springboard.
It is the first time Britain has claimed two medals at a World diving championships and Gallantree becomes the first British female to win a medal at World level.
"We didn't have any expectations ahead of the competition as we know China is so strong and it felt surreal to be competing and, the next thing, we're on top of the podium," said Olympic bronze medallist Daley.
"It's an insane feeling and thanks to Becky for diving so well."
Gallantree said she could not believe she and Daley had won, saying: "We came in for a bit of fun and to practise ahead of our individual events, but it became a bit more than that.
"I never thought I would be a world champion and it's amazing to say now that I am."
Earlier on Wednesday, Olympian Tonia Couch qualified for Thursday's individual 10m platform final and in doing so secured Team GB a place in the event at the Rio Olympics.
The British pair were in third after the first round of an event which featured China's multiple world champion Ruolin Chen and USA's London 2012 10m champion David Boudia.
Two stunning routines by Daley and solid performances from Gallantree saw them move clear of their opponents in the final rounds.
Daley was using the non-Olympic Team event as a warm-up before his individual 10m platform campaign begins on Saturday.
Gallantree, who came 10th alongside Alicia Blagg in the synchronised 3m springboard final on Saturday, will compete in the individual 3m event on Friday.
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.
[NEXT_CONCEPT]
British divers have claimed their second medal of the World Aquatics Championships with Tom Daley and Rebecca Gallantree combining to win gold in the team event.