BridgeCSharpToObjC.cs

using UnityEngine;
using System.Runtime.InteropServices;

public class BridgeCSharpToObjC : MonoBehaviour
{
	#if !UNITY_EDITOR
	[DllImport("__Internal")]
	static extern string Test_ ();
	#endif

	public static string Test ()
	{
		#if !UNITY_EDITOR
		return Test_ ();
		#endif
		return "";
	}

	void Update ()
	{
		Debug.Log (BridgeCSharpToObjC.Test ());
	}
}

TestPlugin.mm

#import <Foundation/Foundation.h>

extern "C"
{
    const char* Test_()
    {
        NSString *sample = @"Fugafuga";
        const char *str = [sample UTF8String];
        // 文字列の場合はmallocが必要!!
        char* result = (char*)malloc(strlen(str)+1);
        strcpy(result, str);
        return result;
    }
}