Charles first met Katherine in New York in 1988, when he was singing with a theatre group and she was a piano accompanist.
"We met quite accidentally outside the rehearsal hall and we fell in love with each other in about two minutes, and that was it," he said.
Charles said he "hit the jackpot" when he met her, adding: "There was no parting us."
Katherine took Charles back to the UK for a visit in 1989, as well as trips to parts of Wales that he admitted he "could not pronounce", including Machynlleth and Llanfairpwllgwyngyll.
"It was just the beginning," said Charles.
"We had a wonderful, wonderful relationship."
At 08:06 EDT, on September 11, 2001, Charles kissed Katherine goodbye from their New York City apartment for the last time.
Exactly 40 minutes later, a jet smashed into the 97th floor of the tower - the very floor she had been working on, in a new job she had started less than a fortnight earlier.
"I knew instantly when it flew over my building - our apartment building - that it was a twin engine jet at full throttle," said Charles, a trained pilot.
"I knew it so well that I called the FBI and told them... 'That was no accident, that was deliberate'."
A year later, when Charles met the FBI agent who took that call, he said the agent told him: "You were the first person to get us on to the fact that that was not an accident."
Katherine had told her husband she was working in "building one". After the first plane hit, news reports described the Twin Towers as "north and south" - and Charles did not know which was which.
But a mile from their home, he could see smoke billowing.
"I couldn't reach her on the phone. Little did I know she ceased to exist the instant that plane hit the building," he said.
Charles "could only assume the worst" until he had confirmation from her employers three days later that nobody had managed to escape above the 91st floor of the north tower - six floors below Katherine's office.
"She didn't know it was going to happen and she never felt a thing," he said.
"In fact... all of her people right there, they were vapourised instantaneously."
He added: "You never knew it was coming - couldn't, not at 450mph (724km/h).
"It was not a blessing that I lost her. It was a blessing that she never knew it happened, and I am grateful for that.
"Not everybody got that. Some people had to make the decision to stay and burn to death or jump from the upper floors."
Last weekend, more than 200 firefighters from across the UK climbed 110 floors - about 2,226 steps - wearing full [personal protective equipment (PPE) in Atlanta, Georgia to commemorate the 25th anniversary of 9/11.
Each of the responders taking part represented one of the 343 firefighters who died while responding to the attacks.
Carl Williams, station manager at Mid and West Wales Fire and Rescue Service, took part in the event along with eight of his colleagues, describing the experience as "quite humbling".
"It was emotional at the end, a lot of people were finishing and crying," he said.
"To achieve that task of just doing the climb, let alone to try and understand the emotion behind it that people have lost their life.
"Nobody expects to go to work and not come home that day."
A documentary reflecting on 25 years since the 9/11 attacks, produced by Newyddion S4C, is available to watch now on YouTube, external.
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