After 14 years of infertility, two losses including a boy, [name_u]Everett[/name_u] [name_m]Lewis[/name_m] [name_m]William[/name_m] at birth, and 7 years of foster care (1 adopted girl, [name_u]Hadley[/name_u] [name_u]Page[/name_u]), we are 7 weeks from expecting the birth of a little girl. [name_u]Everett[/name_u] and [name_u]Hadley[/name_u] were names we fell in love with, the middle names are all family names. We are a teacher and a nurse that have worked extensively with behavioral kids/adults, which has ruined more names than we can count, leaving us with a very short list of names we love, but that we think might be too out there for our general area/community ([name_f]Soleil[/name_f] and [name_f]Saffron[/name_f]) and then we finally came across [name_m]Lachlan[/name_m]/[name_m]Lochlan[/name_m]/[name_f]Locklyn[/name_f]/etc. It is perfect for our Norwegian/Scottish roots and I love the -lyn/n ending as my and my mom’s middle names are [name_u]Lynn[/name_u]. We’re not purists (obviously having a girl [name_u]Hadley[/name_u]), but we don’t want to bastardize the origins too much. What are your thoughts on using the name for a girl and spelling the name? We’d save it for a boy - but this is our last shot (turning 42 and ending the baby dreams on a high note, we hope). Pretty sure middle name will be [name_f]Elaine[/name_f] - another family name.
Behind the name states Lachina as feminine version and [name_f]Lockie[/name_f] as a diminutive of [name_m]Lachlan[/name_m] - I think Lachina [name_f]Soleil[/name_f] [name_f]Elaine[/name_f] will make a good combination
[name_f]Locklyn[/name_f] is probably the best way to spell it. If you’re going to change the spelling at all, may as well make it all phonetic. I don’t love the name, but I think if it’s something that’s special to you then you should go for it. [name_u]Hadley[/name_u] and [name_f]Locklyn[/name_f] pair well for sisters.
I agree with @maerad that [name_f]Locklyn[/name_f] works fine as a sister to [name_u]Hadley[/name_u]!
My first thought in terms of suggestion was [name_u]Lennox[/name_u].
Other ideas ~
[name_f]Annika[/name_f]
Skylark
[name_u]Adair[/name_u]
[name_u]Ainsley[/name_u]
[name_u]Finley[/name_u]
[name_f]Viveca[/name_f]
[name_f]Imogen[/name_f]
[name_u]Dagny[/name_u]
[name_f]Anouk[/name_f]
[name_u]Rowan[/name_u]
[name_m]Rooney[/name_m]
[name_u]Logan[/name_u]
[name_u]Murray[/name_u]
[name_u]Rory[/name_u]
[name_m]Lincoln[/name_m]
[name_u]Collins[/name_u]
I think [name_f]Locklyn[/name_f] and [name_u]Hadley[/name_u] go well together!
I think [name_f]Locklyn[/name_f] is sweet and goes well with [name_u]Hadley[/name_u]!
I think [name_m]Lachlan[/name_m] as a girl’s name is fantastic, beautiful and unique. I’ve thought about adding it to my list, in fact. I would go for it!
[name_f]Locklyn[/name_f] goes super well with [name_u]Hadley[/name_u], and your boy, [name_u]Everett[/name_u].
Yess name her “[name_f]Locklyn[/name_f]” 
I would use Lachina over [name_m]Lachlan[/name_m]/[name_f]Locklyn[/name_f] for a girl.
Why not Lakelyn?
I think [name_f]Soleil[/name_f] has a pretty meaning! I also like [name_f]Saffron[/name_f], it would be perfect in your family. Pick the name you love, it doesn’t matter if it’s too out there for your area 
All the names outside of the top50/100 are too out there for certain people anyways.
I also came to suggest [name_u]Lennox[/name_u].
[name_u]Hadley[/name_u] & [name_u]Lennox[/name_u]?
Not a huge fan of [name_f]Locklyn[/name_f] on a girl, but if you love it, go for it!