I love the nickname [name_u]Kay[/name_u] (K?) but dislike most names started with a K so now I’m trying to find something that has a similar sound. Any ideas what can I use?
Thanks, Berries! 
[name_f]Catherine[/name_f], [name_f]Cathleen[/name_f], [name_f]Caitlin[/name_f], and their variants would work, since these are the traditional long forms of [name_u]Kay[/name_u].
[name_f]Michaela[/name_f] is another logical option.
[name_f]Cadence[/name_f]
Caelia
[name_f]Calliope[/name_f]
[name_f]Cassiopeia[/name_f]
[name_f]Caroline[/name_f]
Could always just use [name_u]Kay[/name_u] itself? Maybe a name like [name_f]Michaela[/name_f]? which has the kay pronounced within it.
Obvious [name_u]Kay[/name_u] beginnings…
[name_f]Cayla[/name_f]- I know two with their name spelled this way and none with a K
[name_f]Cady[/name_f]
These have a kah beginning…
[name_f]Callie[/name_f]
[name_f]Calla[/name_f]
[name_f]Callista[/name_f]
These are my favorite [name_f]Catherine[/name_f]/[name_f]Caitlin[/name_f] variations… [name_f]Cate[/name_f]/[name_f]Cat[/name_f] works as a nickname so I can see [name_u]Kay[/name_u] working as well…
[name_f]Caterina[/name_f]
[name_f]Catalina[/name_f]
[name_f]Kayla[/name_f], [name_f]Katherine[/name_f]/[name_f]Catherine[/name_f], [name_f]Kathleen[/name_f], [name_f]Caitlin[/name_f]/[name_f]Kaitlin[/name_f] etc.
[name_f]Acacia[/name_f] maybe?
[name_u]Kay[/name_u] is a lovely name all by itself…
I love [name_u]Kay[/name_u] as a stand alone name, but some options are (basically any ‘ca’ or ‘ka’ starting names):
With the obvious ‘K’ sound:
[name_u]Kato[/name_u] / [name_m]Cato[/name_m], [name_f]Acacia[/name_f], [name_f]Cadence[/name_f], [name_f]Leocadia[/name_f], [name_f]Arcadia[/name_f], [name_f]Acadia[/name_f], [name_f]Arcadie[/name_f], etc.
Could also work just as well:
Kazue, [name_f]Calais[/name_f], [name_f]Katherine[/name_f] / [name_f]Catherine[/name_f], [name_f]Katia[/name_f] / [name_f]Katya[/name_f], [name_f]Catalina[/name_f], [name_f]Catrina[/name_f] / [name_f]Catriona[/name_f], [name_f]Catrine[/name_f], [name_f]Kailani[/name_f], [name_f]Kalila[/name_f], [name_f]Callista[/name_f], [name_f]Callisto[/name_f], [name_f]Calliope[/name_f], [name_f]Camellia[/name_f], [name_f]Camille[/name_f], [name_f]Camilla[/name_f], [name_f]Caroline[/name_f], [name_f]Carolina[/name_f], [name_f]Carys[/name_f], [name_f]Cassia[/name_f], [name_f]Cassiopeia[/name_f], [name_f]Katenka[/name_f], [name_f]Aikaterine[/name_f], [name_f]Carine[/name_f], [name_f]Katalin[/name_f], [name_f]Katica[/name_f], [name_f]Ecaterina[/name_f], [name_f]Kazuko[/name_f], [name_u]Cairo[/name_u], [name_f]Calia[/name_f], [name_f]Cadenza[/name_f], etc.
My favourites for [name_u]Kay[/name_u] (which I love) are [name_f]Katharine[/name_f], [name_f]Ketzia[/name_f], [name_f]Kathleen[/name_f], [name_f]Katia[/name_f], [name_f]Kelilah[/name_f], and Kalanit in roughly that order. I think [name_u]Kay[/name_u] for a name that doesn’t start with K is unnecessarily jarring/confusing but I’d go with:
[name_f]Acacia[/name_f]
Arkadia
[name_f]Leokadia[/name_f]
I think if you go with [name_f]Catherine[/name_f]/[name_f]Caitlin[/name_f]/[name_f]Cadence[/name_f] and call her [name_u]Kay[/name_u], people will assume the long forms are [name_f]Katherine[/name_f], [name_f]Katelyn[/name_f] etc., and [name_f]Kadence[/name_f].