It was a new unionist political party that according to its strapline would "aspire to better" for Northern Ireland.
But within a year it imploded amid a chaotic public fallout on the eve of an election and allegations of bullying and sexual misconduct.
Ten years ago NI21 was officially launched by its two ambitious founders, then MLAs Basil McCrea and John McCallister.
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It was six months after the Union flag protests and Brexit was nowhere in sight.
The pair had quit the Ulster Unionist Party earlier that year over a decision to field a joint candidate with the DUP in a Westminster by-election.
Mr McCallister recalled it was a "scorching hot day" as they launched NI21 on June 6 2013 at the Mac in Belfast's Cathedral Quarter.
"There was certainly an air of excitement," he said.
In founding NI21, he said they hoped to represent people who "just wanted Northern Ireland to work".
They aimed to be a "pro-Union party that didn't need flags and bunting and pictures of the Queen hanging up everywhere".
"'Pro-Union' was the term because 'unionism' even we felt 10 years ago was slightly off-putting for people who were content with the status quo."
NI21 continued to attract interest after its launch, with Mr McCallister saying hundreds gathered at the Europa Hotel for their first party conference in November that year.
Less than 12 months after launching, the party faced their first test at the ballot box with the European Parliament and council elections held in May 2014.
NI21's candidate for an MEP seat was party chairwoman Tina McKenzie, who also stood in the local government election among 48 candidates on their ticket.
Some billboards for her campaign used the Irish language in an attempt to reach out to the electorate.
But just two days before the election, the party descended into chaos.
Mr McCallister publicly objected to the party's decision to switch from designating as "unionist" in the Stormont Assembly to "other".
Allegations also surfaced against Mr McCrea of bullying and sexual misconduct towards his staff.
A Stormont standards commissioner later investigated and in 2016 dismissed all complaints against him.
But MLAs on the Assembly's standards committee said his treatment of his staff occasionally fell short of what they would expect.
They said he exercised poor judgement in allowing young women into his hotel rooms and criticised how he spoke to his staff in some instances.
Mr McCrea described the complaints against him as a "set-up" and an "attempted political assassination".
Reflecting on the controversy that engulfed the party, Mr McCallister said it was a "dreadful period for pretty well everyone involved".
He added: "For everyone we brought into politics and got excited about politics being new and fresh, it really didn't look new and fresh. It looked same old, same old."
The solitary electoral success for NI21 was Johnny McCarthy, who narrowly won a seat on Lisburn and Castlereagh City Council.
"It was a bit of a whirl that day. To be honest I wasn't going to show up at the count because of everything that was going on," he said.
Mr McCarthy recalled how, with little expectation of being elected, he decided he was not going to wear a suit.
"That's why in the middle of May there's these pictures of me wearing this Christmas jumper," he said.
Mr McCallister quit the party and continued as an independent MLA, but failed to be re-elected in 2016. He now works in the agriculture sector.
Mr McCrea remained leader of NI21 but did not seek re-election and the party later disbanded.
Mr McCarthy briefly succeeded Mr McCallister as deputy leader but quit in 2015 and later joined the SDLP.
The former councillor said there were "a lot of great people" involved in NI21 "who did want to try and make something of it".
"There was a core of us who tried to rebuild but I suppose the initial excitement dissipated very swiftly because of everything that went on. It became very difficult," he said.
Ten years on, Mr McCallister said he does not regret NI21, arguing the party was "calling some of the big issues right".
With UUP leader Doug Beattie now attempting to shift his party in a more liberal direction, some believe NI21 was ahead of its time.
But a surge in support for Alliance in recent years has led some to argue the Ulster Unionists and political unionism have been too slow to move.
Mr McCallister said the UUP "should have gone on that journey sooner".
"There has to be that more liberal unionist, pro-Union party. Or else Alliance becomes that comfortable alternative," he said.
Mr McCarthy did not seek re-election this year due to work commitments with a new job in the private sector.
He said the shift in politics in the past decade has demonstrated the need for a "progressive voice" which NI21 had hoped to achieve.
"What NI21 did was bring people out who were not connected with politics in any way, shape or form. It brought out good people. It brought them into politics who wouldn't have been before."
He said the idea of "Northern Ireland plc" has been picked up by Alliance, while instead unionism appears to have been "very protectionist of what unionism is".
But he added: "The opportunity for a new party is maybe unfortunately hamstrung by what happened with NI21."
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