Remove the sim lock from the Samsung Galaxy S2 LTE SC-03D - Japan Docomo

When I did this I had stock 4.0.4 (SC03DOMMP5 / SC03DDCMMP5) flashed with ODIN from sammobile.com (you can do it with heimdall too I guess). *Very important* 

In order to sim unlock the SC-03D it is necessary to first flash a "special" radio. Flashing the radio is possible with a CWM based recovery. Get the latest version of CWM-recovery from this forum and flash it using ODIN/Heimdall:
http://android.slateblue.tk/viewtopic.php?f=20&t=73

Then get the radio to flash:
Official TMo ICS RADIO_NOTE.zip
(from this XDA thread:
http://forum.xda-developers.com/showpost.php?p=37564563)

This radio is from the Galaxy Note (SC-05D) so it made my phone very very slow. However, you need this radio because it has options to make the phone sim free. Options which the original SC-03D radio does not. As soon as you flash this new radio boot the phone, run the dialer application and dial:

*#9090#

it might be very slow but eventually you should get to the service menu.

To access more options press [menu] then [Input key]. Then into the text box type:

Q0   (Q Zero, not QOh)


It should bring you to another menu after a second or so. Select these options:

[1] DEBUG SCREEN

[8] PHONE CONTROL

[5] SIMULATION

[6] NETWORK LOCK

[3] PERSO SHA256 OFF

Wait for 30 seconds , press MENU then BACK.

Select:
[4] NW LOCK NV DATA INITIALLIZ

Wait 60 seconds. Reboot.

Congrats you're sim free. You should remain sim free no matter what radio/firmware/rom you flash.

partial credit to FAB: 
http://blogs.yahoo.co.jp/fab4_boys/26754215.html
And HomuHomu for the recovery.

How to JailBreak the Samsung Galaxy S2 sc-02c Docomo Japan

I assume you have a stock SC-02C by docomo. If you don't, download the firmware from sammobile and flash with ODIN.

Then flash Siyah Kernel 5.0.1 for S2 with ODIN:
http://d-h.st/Bcf

Hold Home+Volume Up+Power to go into recovery mode. Wipe Data/System.
Reboot into your rooted SC-02c.

If you need to break the sim lock of the phone copy this apk to the phone and install it. It will remove the sim lock:

http://d-h.st/6EfJ

After you have ensured SU exists and have used some manager to remove the junk apps by docomo re-flash the original kernel by extracting the zImage from the samsung firmware tar file and flashing the kernel only (careful don't reflash the entire firmware because you'll end up doing everything from scratch).

Re-flashing the original kernel is important to get the GPS working, I couldn't get WjChen0's gps fix to work (by adding it under /etc/init.d)

Good luck.