The final 20:

I only need 20 more characters in order to assign every 5-star song an associate.  If I relax the standards a bit this shouldn't be impossible, so let's go:

Ghost in the Shell's Motoko -- she gets Inner Universe because it's a Ghost in the Shell song.  That means Chihiro from Spirited Away needed a new song, so she got Toto'haha Island instead.  Motoko has been in '100 Waifus' before so this reinstatement is pretty obvious.

Ika from Squid Girl -- she gets Squid Girl's theme song, HIGH POWERED, which leaves Miku without a song.  Miku can have An Earnest Desire of Grey.  Ika was previously in '100 Waifus' so no big deal there.

Youko from 12 Kingdoms -- again someone already in '100 Waifus' before.  She gets Cloak and Dagger.

Iria from Iria: Zeiram the Animation --  I don't like the show Iria much anymore, but Iria herself is a beautiful, strong, cool girl.  She gets Memories of the World.

Jubei from Ninja Scroll -- Nothing like a skilled samurai to get the blood pumping.  He can get Armistead Is Hit.

Aya from Tenjou Tenge -- She's hot, loving and a skilled warrior, good enough for me.  She gets the Tenjou Tenge theme song, Bomb A Head! V.  Which doesn't lower my unused song count because it's not 5-star.

Yuzuriha from X -- X is certainly a beautiful series if nothing else, and Yuzuriha is the most relatable character out of the panoply. She's a beastmaster who wields a cool dog spirit to fight alongside, like Nakoruru.  She can have Fife and Gun.

Karna from Fate/Apochrypha -- Karna is the coolest summon from Fate/Apochrypha.  He gets a Fate/etc. song, which is his due, Oath Sign.  Hisame used to have Oath Sign, so he was given a new theme, March to Mortality (Pickett's Charge).

Arshes, Yoko and Gaara from Bastard! -- Arshes' feelings for Darsh while she tragically dueled him were enough to make me cry in the original oav's.  Gaara also fought remarkably hard for the good guys once he changed sides, and Yoko is a good match for Dark, as someone needs to rein him in.  Gaara gets The First Battle.  Arshes gets Battle at Devil's Den.  Yoko gets Battle of Little Round Top.

Usagi Drop -- Rin was previously in '100 Waifus' so she's an obvious include.  She was adorable as a child and I have no objection to her 'step-incest' as an adult.  Rin gets Gazing Over the Great Plain.

Azumanga Daioh -- Ayumu ('Osaka') and Chiyo are both cute and funny from this gag series.  I believe Ayumu featured in '100 Waifus' before eventually being replaced.  Ayumu gets The Highlands.  Chiyo gets Atlasdam, Seat of Learning.

Tears to Tiara -- Hamilcar, Astarte, Charis, Riannon, and Rathy are all worthy includes.  Astarte was in '100 Waifus' for a long time before finally being replaced.  Hamilcar gets General Lee's Solitude.  Astarte gets Crusade of the Gods.  Charis gets South Valley (Port Noir Maps Battle).  Riannon gets Scholar Oswald's Theme.  Rathy gets Over the Fence.

Death Note -- Misa gets Killer Angel.  She's pretty, devoted, and fought for the right side.

Rave Master -- Celia gets The Cliftlands.  This mermaid was the best part of Rave Master, minor character notwithstanding.

That's it!  That's everyone!  Every song accounted for!  And crazily enough, a lot of these characters were in '100 Waifus' before they weren't, meaning they're still wonderful grade.   I didn't have to lower my standards much at all.  It's also convenient that the total number of honored characters is a nice round 1400.   Another giant project complete.