Kairi never had a need to use one nor experience with a sword enough to know how to use her Keyblade. Kairi's heart was stored inside Sora when the Heartless attacked Destiny Island, and she was kept in a comatose state where she couldn't do anything. When her heart found its way back into her, she was under the care of other characters who were more experienced than her in fighting, and come Kingdom Hearts 2, she (because of Sora's story in Castle Oblivion) had forgotten all about Sora and the attack on Destiny Islands, making her assume that there wasn't a need for her to use or learn how to use the Keyblade. When Axel takes her, she is imprisioned by Saïx and Naminé lets her escape. They encounter Riku who gives Kairi a keyblade of her own so she can defend herself, and that was the first time Kairi ever truly needed a keyblade. Even though she already had inherited one from Aqua, Kairi's life, unlike Sora's and Riku's never gave her much space for her to awaken to it and learn how to use it.
As for Riku, Riku had inherited Terra's keyblade, but because Riku had succumbed to darkness, the keyblade instead went to the closest person with a heart and skill strong enough to wield it, that being Sora. This is why Riku is able to use the Keyblade temporarilly in Hollow Bastion. It's not known how he got it, but by the time Sora reaches the World that Never Was, Riku starts using a keyblade called "Way to the Dawn", fashioned after his Soul Eater sword.
As for Isa, he was never seen with a weapon other than the Claymore. Like Braig, he probably doesn't have a keyblade of his own, but due to Xehanort's heart being inside him now, he might gain the ability to use a Keyblade as well.