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Cooper-Lesadd said she had approached Plaid Cymru after a "period of deliberation".
"It's been a journey for me since coming to the Senedd, it's come after a period of thinking about it, so it has been a while," she said.
She described speeches by Reform Caerdydd Penarth MS Joe Martin in June, including one making allegations about Sudanese asylum seekers, as "flashpoints" for her.
In the speech he implied asylum seekers from the country posed a safety risk.
Martin's words, Cooper-Lesadd said, "just completely did not chime with the type of person that I am and the values that I have".
"There were other several things. The party's completely chaotic, it's falling apart at the seams, it really, really is."
Cooper-Lesadd, a former Conservative, was elected as a Reform MS for the Pen-y-bont Bro Morgannwg constituency in May's election.
Her swap to Plaid Cymru means she has been in three political parties since 2024.
She said she felt uncomfortable with some of the language used by Reform members.
During the interview, Cooper-Lesadd was also asked about her view on independence, and further powers for the Welsh government, two things that Plaid Cymru has campaigned on.
"I support the ability of people to choose their own future, and that is self-determination and Plaid have said, that it will happen when the time is right.
"I would vote yes, because, I have seen increasingly the fact that Wales is short-changed in so many different ways, but we aren't there yet.
"The nation is on a political journey, and I'm on a political journey as well, and it's about bringing people with us."
The new Plaid MS was also asked about the idea of the UK returning to the European Union.
"I've never, ever spoken about Brexit, and I'll be honest I voted remain in the referendum and it's been quite difficult for me not to be able to talk about it, and it has damaged our trading relationships. So yes, I absolutely do support that.
"In the public discourse over the last 10 years, we haven't been able to have that debate about the European Union. Even parties such as the Labour Party have struggled to talk about it, because they just get shut down."
"When I came into Reform, I did think that there was a certain element of being a broad church, and that's what I believed.
"It has become extremely narrow, whereas Plaid has a set of values that is values-based, and that's what I believe in."
The Senedd, in common with the other parliaments in the UK, has no requirement for members to face a by-election if they leave the party they represented when they were elected.
"What I would say is that my work with my constituents doesn't end today, I'll continue fighting for them, continue standing up for them.
"I appreciate [Reform's point of view], but I'm going to keep on fighting for people in my area and working extremely hard."
Reform MS Llyr Powell told BBC Radio Wales Breakfast on Wednesday that the party had "made a lot of effort to try to communicate" with Cooper-Lesadd prior to her decision to leave Reform.
He questioned whether Cooper-Lesadd "fully understood the core values of the party that she was standing for" and the reasons given for her defection.
